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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Mar. 02, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	0001590877
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		31,655,093
Entity Public Float			$ 505.0

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 46,656	$ 24,840
Marketable securities	114,122	64,714
Accounts receivable	473	1,032
Prepaid expenses	5,334	1,775
Other current assets	1,412	1,010
Total current assets	167,997	93,371
Marketable securities	15,616	69,412
Property and equipment, net	13,977	9,324
Restricted cash	225	225
Other assets	862	400
Total assets	198,677	172,732
Current liabilities
Accounts payable	4,832	1,543
Accrued expenses and other current liabilities	9,605	8,126
Total current liabilities	14,437	9,669
Deferred rent, net of current portion	1,211	1,326
Total liabilities	15,648	10,995
Commitments and contingencies (Note 6)
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at December 31, 2017 and December 31, 2016
Common stock; $0.0001 par value; 100,000 shares authorized at December 31, 2017 and December 31, 2016; 31,295 and 26,477 shares issued and outstanding at December 31, 2017 and December 31, 2016, respectively	3	3
Additional paid-in capital	371,497	276,354
Accumulated other comprehensive loss	(715)	(33)
Accumulated deficit	(187,756)	(114,587)
Total stockholders’ equity	183,029	161,737
Total liabilities and stockholders’ equity	$ 198,677	$ 172,732

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	31,295,000	26,477,000
Common stock, shares outstanding	31,295,000	26,477,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
License revenue	$ 10,385	$ 4,303	$ 5,025
License revenue from related party			2,000
Reagent sales		213	257
Grant revenue	8	73	306
Total revenues	10,393	4,589	7,588
Costs of revenues
Licensing costs	1,703	861	1,405
Costs of reagent sales	6	98	98
Research and development	57,224	45,482	17,279
General and administrative	27,229	23,590	11,912
Other operating expenses (income)	116	(102)	31
Total operating expenses	86,278	69,929	30,725
Loss from operations	(75,885)	(65,340)	(23,137)
Other Income (Expense)
Investment income	2,716	1,938	346
Interest expense			(20)
Total other income (expense)	2,716	1,938	326
Loss before income taxes	(73,169)	(63,402)	(22,811)
Income Tax Benefit	0	435	0
Net loss	(73,169)	(62,967)	(22,811)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(682)	686	(719)
Total other comprehensive income (loss)	(682)	686	(719)
Comprehensive loss	(73,851)	(62,281)	(23,530)
Reconciliation of net loss to net loss applicable to common stockholders
Net loss	(73,169)	(62,967)	(22,811)
Net accretion and dividends on convertible preferred stock			(1,747)
Net gain on extinguishment of convertible preferred stock			759
Net loss applicable to common stockholders	$ (73,169)	$ (62,967)	$ (23,799)
Basic and diluted net loss per common share	$ (2.45)	$ (2.38)	$ (2.59)
Weighted-average basic and diluted common shares	29,878	26,409	9,173

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]
Balances at Dec. 31, 2014						$ 3,963	$ 8,630			$ 12,593
Balances (Shares) at Dec. 31, 2014						2,393,000	1,906,000
Balances (Shares) at Dec. 31, 2014		2,645,000
Balances at Dec. 31, 2014	$ (18,291)		$ 10,518		$ (28,809)
Issuance of stock, net of transaction costs	145,184	$ 1	145,183					$ 26,021		26,021
Issuance of stock, net of transaction costs, shares		7,245,000						4,047,000	7,367,000
Issuance of convertible preferred stock for the conversion of outstanding related party debt								$ 3,792		3,792
Issuance of convertible preferred stock for the conversion of outstanding related party debt, Shares								585,000
Issuance of convertible preferred stock, net of transaction costs									$ 67,998	67,998
Loss (gain) on extinguishment of convertible preferred stock	759		759			$ 1,317	$ (2,076)			(759)
Accretion (decretion) of convertible preferred stock	(1,747)		(1,747)			(2,280)	1,338	$ 187	2,502	1,747
Conversion of convertible preferred stock into common stock upon initial public offering	111,392	$ 2	111,390			$ (3,000)	$ (7,892)	$ (30,000)	$ (70,500)	$ (111,392)
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		16,298,000				(2,393,000)	(1,906,000)	(4,632,000)	(7,367,000)
Discount on related party promissory notes	13		13
Exercise of stock options	107		107
Exercise of stock options, Shares		125,000
Stock-based compensation expense	2,921		2,921
Unrealized loss on available-for-sale securities	(719)			$ (719)
Net loss	(22,811)				(22,811)
Balances (Shares) at Dec. 31, 2015		26,313,000
Balances at Dec. 31, 2015	216,808	$ 3	269,144	(719)	(51,620)
Exercise of stock options	179		179
Exercise of stock options, Shares		163,000
Stock-based compensation expense	7,031		7,031
Unrealized loss on available-for-sale securities	686			686
Net loss	$ (62,967)				(62,967)
Balances (Shares) at Dec. 31, 2016	0
Balances (Shares) at Dec. 31, 2016		26,477,000
Balances at Dec. 31, 2016	$ 161,737	$ 3	276,354	(33)	(114,587)
Issuance of stock, net of transaction costs	81,489		81,489
Issuance of stock, net of transaction costs, shares		4,255,000
Exercise of stock options	2,493		2,493
Exercise of stock options, Shares		516,000
Issuance of common stock under employee stock purchase plan	556		556
Issuance of common stock under employee stock purchase plan, shares		48,000
Stock-based compensation expense	10,605		10,605
Unrealized loss on available-for-sale securities	(682)			(682)
Net loss	$ (73,169)				(73,169)
Balances (Shares) at Dec. 31, 2017	0
Balances (Shares) at Dec. 31, 2017		31,295,000
Balances at Dec. 31, 2017	$ 183,029	$ 3	$ 371,497	$ (715)	$ (187,756)

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Series C Convertible Preferred Stock [Member]
Issuance of Stock, transaction costs		$ 187
Series D Convertible Preferred Stock [Member]
Issuance of Stock, transaction costs		2,502
Common Stock [Member]
Issuance of Stock, transaction costs	$ 5,738	$ 14,206

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (73,169)	$ (62,967)	$ (22,811)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	10,605	7,031	2,921
Net amortization of premiums and accretion of discounts on marketable debt securities	1,815	2,004	311
Depreciation and amortization	2,686	544	80
Net realized gains on sales of marketable securities	(479)
Non-cash consideration received for licenses granted	(420)
Other non-cash adjustments	73	(399)	24
Changes in operating assets and liabilities
Accounts receivable	561	1,073	(265)
Prepaid expenses	(3,559)	(755)	(992)
Other current assets	(402)	(159)	(851)
Other assets	(135)	(139)	(168)
Accounts payable	2,621	186	590
Accrued expenses and other current liabilities	1,897	3,873	2,221
Due to related party under services agreement			(34)
Other related party payables			(3,761)
Advance payments		(127)	(26)
Deferred rent	(86)	1,277	246
Net cash used in operating activities	(57,992)	(48,558)	(22,515)
Cash flows from investing activities
Purchases of marketable securities	(68,634)	(45,072)	(163,278)
Maturities of marketable securities	70,224	72,586
Sales of marketable securities	780	23
Purchases of property and equipment	(7,160)	(8,149)	(522)
Restricted cash		(225)
Net cash provided by (used in) investing activities	(4,790)	19,163	(163,800)
Cash flows from financing activities
Proceeds from exercise of stock options	2,493	179	107
Proceeds from issuance of common stock under employee stock purchase plan	556
Proceeds from public offering of common stock, net of underwriting discounts and commissions	81,994
Proceeds from initial public offering of common stock, net of transaction costs			145,184
Issuance costs for public offering of common stock	(445)	(60)
Net cash provided by financing activities	84,598	119	239,310
Net increase (decrease) in cash and cash equivalents	21,816	(29,276)	52,995
Cash and cash equivalents
Beginning of period	24,840	54,116	1,121
End of period	46,656	24,840	54,116
Supplemental cash flow information
Cash paid for interest			7
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses	254	1,181	96
Non-cash consideration received for licenses granted	$ 420
Issuance costs for public offering of common stock in accounts payable and accrued expenses		$ 33
Conversion of related party promissory notes into Series C convertible preferred stock			1,389
Conversion of convertible preferred stock into common stock upon initial public offering			111,392
Series C Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs			26,021
Supplemental disclosures of non-cash investing and financing activities
Conversion of accrued service fees to related party			2,403
Series D Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs			$ 67,998

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) is a leading clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. The Company’s proprietary adeno-associated virus (AAV) gene delivery platform (NAV® Technology Platform) consists of exclusive rights to over 100 novel AAV vectors, including AAV7, AAV8, AAV9 and AAVrh10. The Company’s NAV Technology Platform is being applied by the Company, as well as by third-party licensees (NAV Technology Licensees), in the development of product candidates for a variety of diseases with unmet needs. The Company was formed in 2008 in the State of Delaware and is headquartered in Rockville, Maryland. Initial Public Offering On September 22, 2015, the Company completed its initial public offering (IPO) in which it sold 7,245,000 shares of common stock (inclusive of 945,000 shares of common stock sold by the Company pursuant to the full exercise of an option to purchase additional shares granted to the underwriters in connection with the offering) at a price of $22.00 per share. The shares began trading on The Nasdaq Global Select Market on September 17, 2015. The aggregate net proceeds received by the Company from the offering were $145.2 million, net of underwriting discounts and commissions and offering expenses payable by the Company. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 16,298,045 shares of common stock. Follow-on Public Offering On March 27, 2017, the Company completed a follow-on public offering in which it sold 3,700,000 shares of common stock at a price of $20.50 per share. In connection with the offering, the Company granted the underwriters an option to purchase up to 555,000 additional shares of common stock at the public offering price. The underwriters exercised the option in full and purchased the additional shares on April 26, 2017. The aggregate net proceeds received by the Company from the offering, inclusive of the underwriters’ option exercise, were $81.5 million, net of underwriting discounts and commissions and offering expenses payable by the Company. Liquidity and Risks As of December 31, 2017, the Company had generated an accumulated deficit of $187.8 million since inception. As the Company continues to incur losses, transition to profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support the Company’s cost structure. The Company may never achieve profitability, and unless and until it does, the Company will continue to need to raise additional capital. As of December 31, 2017, the Company had cash, cash equivalents and marketable securities of $176.4 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these consolidated financial statements were issued. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, development by the Company or its competitors of technological innovations, risks of failure of clinical trials, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to transition from clinical manufacturing to the commercial production of products.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB).

The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Foreign Currency Transactions

The functional currency of the Company and its consolidated subsidiaries is the U.S. dollar. Transaction gains and losses that arise from exchange rate fluctuations on transactions denominated in a currency other than the U.S. dollar are included in the consolidated statements of operations and comprehensive loss as incurred.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements. Estimates are used in the following areas, among others: stock-based compensation expense, accrued research and development expenses and other accrued expenses, income taxes and the fair value of financial instruments.

Reclassifications

Certain amounts reported in prior periods have been reclassified to conform to current period financial statement presentation. These reclassifications are not material and have no effect on previously reported financial position, results of operations and cash flows.

Segment and Geographical Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker (CODM), or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s CODM, the Chief Executive Officer, views its operations and manages its business as one operating segment.

For the year ended December 31, 2017, 96% of the Company’s revenue was generated from customers located in the U.S. For the year ended December 31, 2016, 90% of the Company’s revenue was generated from customers located in the U.S. For the year ended December 31, 2015, 78% and 21% of the Company’s revenue was generated from customers located in the U.S. and Europe, respectively. Country of origin for revenue is determined by the Company based on the country of domicile of licensees for license revenue, the country in which reagent sales are delivered to for reagent sales and the location of grantors for government grant revenue. The substantial majority of the Company’s assets currently reside in the U.S.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and money market mutual funds.

Restricted Cash

Restricted cash includes money market mutual funds used to collateralize an irrevocable letter of credit as required by the Company’s lease agreement for its office space in New York, New York.

Marketable Securities

The Company’s marketable securities are classified as available-for-sale and are carried at fair value. Marketable securities with remaining maturity dates exceeding 12 months which are not intended to be sold prior to maturity for use in current operations are classified as non-current. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive income (loss) and reported as a separate component of stockholders’ equity until realized. A decline in the fair value below cost of available-for-sale securities that is deemed other-than-temporary is charged to results of operations, resulting in the establishment of a new cost basis for the security. Premiums and discounts are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Realized gains and losses from the sale of marketable securities are based on the specific identification method and are included in results of operations.

The Company regularly evaluates whether declines in the fair value of its investments below their cost are other-than-temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. If the Company determines that the decline in fair value of an investment is below its accounting basis and this decline is other-than-temporary, the Company would reduce the carrying value of the security it holds and record a loss for the amount of such decline. The Company has not recorded any declines in value judged to be other-than-temporary on its investments.

Concentrations of Credit Risk and Off-balance Sheet Risk

Cash and cash equivalents, marketable securities and accounts receivable are financial instruments that are potentially subject to concentrations of credit risk. The Company’s cash and cash equivalents are deposited in accounts at multiple financial institutions, and amounts may exceed federally insured limits. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company’s marketable securities primarily consist of fixed income corporate debt securities and may subject the Company to concentrations of credit risk. The Company has adopted an investment policy which limits potential concentrations of investments and establishes minimum acceptable credit ratings, thereby reducing credit risk exposure. Management believes that it is not exposed to significant credit risk related to accounts receivable due to the credit quality and history of collections from its significant customers. The Company has no financial instruments with off-balance sheet risk of loss.

The following table summarizes those customers who represented at least 10% of revenue or accounts receivable for the periods presented:

Revenue

Accounts Receivable

Years Ended December 31,

December 31,

2017

2016

2015

2017

2016

Customer A

Customer B

Customer C⁽¹⁾

Customer D

Customer E

Customer F

Customer G

Customer H

Customer I

*	Represented less than 10%

(1)	Represents a related party prior to September 30, 2015

Accounts Receivable

Accounts receivable consist of amounts due to the Company resulting from the Company’s licensing arrangements, reagent sales and grant programs. Receivables are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability using specific identification of account balances and historical information regarding write-offs. Account balances are charged off against the allowance when the potential for recovery is considered remote. The Company has not recorded an allowance for doubtful accounts as of December 31, 2017 or 2016.

Property and Equipment

Property and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:


Computer equipment and software			3 years
Lab equipment			5 years
Furniture and fixtures			5 years
Leasehold improvements			Shorter of lease term or estimated useful life

Impairment of Long-lived Assets

The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No impairment losses have been recorded during the years ended December 31, 2017, 2016 and 2015.

Cost Method Investments

Cost method investments consist of non-marketable equity holdings and are stated at cost. The Company accounts for its investments in other entities using the cost method if its ownership interest is below 20% and the Company does not have significant influence over the operations of the entity. As of December 31, 2017, cost method investments consisted of shares of common stock of Prevail Therapeutics Inc. (Prevail) acquired in connection with a commercial license granted to Prevail in August 2017. Cost method investments had a carrying value of $0.4 million as of December 31, 2017 and are included in other assets on the consolidated balance sheet. The Company did not have any cost method investments as of December 31, 2016.

Declines in the fair value of cost method investments below their carrying value that are deemed to be other-than-temporary are reflected in the consolidated statements of operations and comprehensive loss as realized losses. In estimating other-than-temporary impairment losses, management considers, among other things, the length of time and the extent to which the fair value has been less than cost, the financial condition and near-term prospects of the issuer and the intent and ability of the Company to retain its investments in the issuer for a period of time sufficient to allow for the anticipated recovery in fair value. The Company has not identified any events or changes in circumstances that would have an adverse effect on the fair value of its cost method investments. Accordingly, no other-than-temporary impairment losses were recorded for the years ended December 31, 2017, 2016 and 2015.

Fair Value of Financial Instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurements and Disclosures (ASC 820), establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:

•

Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

•

Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

•

Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments.

Revenue Recognition

The Company primarily generates revenue through license agreements with third parties which may grant rights to the research, development, and commercialization of product candidates using the Company’s NAV Technology Platform. Additionally, the Company has generated limited revenue from sales of licensed reagents to customers for use in research and development as well as grant programs.

The Company recognizes revenue in accordance with FASB ASC Topic 605, Revenue Recognition (ASC 605). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met:

•

Persuasive evidence of an arrangement exists;

•

Delivery has occurred or services have been rendered;

•

The seller’s price to the buyer is fixed or determinable; and

•

Collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s balance sheets.

The Company analyzes its revenue arrangements based on the guidance in FASB ASC Topic 605-25, Revenue Recognition-Multiple-Element Arrangements (ASC 605-25). Pursuant to the guidance in ASC 605-25, the Company evaluates multiple-element arrangements to determine (i) the deliverables included in the arrangement and (ii) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. This evaluation involves subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that: (i) the delivered item(s) has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially in the control of the Company. The Company does not have any material revenue arrangements that contain multiple deliverables.

License Revenue. The terms of the Company’s license agreements require delivery of an intellectual property license for use of the Company’s intellectual property in research and/or commercial development of product candidates for various diseases. License agreements generally have a term equal to the life of the intellectual property, but are terminable at the option of the licensee. Non-refundable payments to the Company under these arrangements may include: (i) up-front license fees, (ii) option fees to obtain additional commercial licenses, (iii) annual maintenance fees, (iv) sublicense fees, (v) payments based on the achievement of certain milestones based solely on the efforts of the licensees and (vi) royalties on product sales.

Nonrefundable up-front license fees are recognized as revenue upon delivery of the license provided there are no undelivered elements in the arrangement and the necessary criteria under ASC 605 for revenue recognition have been met.

Options to exercise commercial licenses are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the licensee will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the licensee might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. For arrangements under which an option is considered substantive, the Company does not consider the item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration, provided the option is not priced at a significant and incremental discount. Conversely, for arrangements under which an option is not considered substantive or if an option is priced at a significant and incremental discount, the Company would consider the item underlying the option to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration. As of December 31, 2017 and 2016, all of the options included in the Company’s license agreements have been determined to be substantive, and none of the options are priced at a significant and incremental discount. Option fees are recognized as revenue upon exercise and delivery of the underlying commercial license, provided there are no undelivered elements in the arrangement and the necessary criteria under ASC 605 for revenue recognition have been met.

Annual maintenance fees under the Company’s license agreements do not represent a separate deliverable aside from the delivery of the license since the Company has no further obligations under the agreements. Accordingly, annual maintenance fees are recognized as revenue when billable under the agreement, provided the price is fixed or determinable and collectability is deemed reasonably assured.

Sublicense fees are payable to the Company upon the receipt of certain fees by the licensee from any sublicensees. Sublicense fees received by the Company are recognized as revenue when the price is fixed or determinable and collectability is deemed reasonably assured.

At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone and the level of effort and investment required to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. The Company has concluded that all of the clinical, regulatory and commercial milestones pursuant to its license agreements are substantive. Milestone payments are recognized as revenue upon achievement of the milestone by the licensee, provided that all other revenue recognition criteria are satisfied.

The Company recognizes royalty revenue, if any, in the period of sale of the related product(s) based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, and provided that all other revenue recognition criteria under ASC 605 are satisfied. To date, the Company has not generated any royalty revenues.

Please refer to Note 12 for information regarding license revenue from related party.

Reagent Sales. Reagent sales consist of sales of licensed reagents to third parties for use in research and development. Revenue from reagent sales is recognized upon delivery to customers, provided that all other revenue recognition criteria under ASC 605 are satisfied.

Grant Revenue. Grant revenue is generated through research and development grant programs offered by the U.S. federal government and the European Union. Government grants provide funds for certain types of expenditures in connection with research and development activities over a contractually defined period. Revenue related to government grants is recognized in the period during which the related costs are incurred and the related services are rendered, provided that the applicable performance obligations under the grants have been met. Funds received under grants are recorded as revenue if the Company is deemed to be the principal participant and primary obligor in the contract arrangements because the activities under the contracts are part of the Company’s development programs. If the Company is not the principal participant or primary obligor, the grant proceeds are recorded as a reduction to research and development expense.

The Company’s grants contain refund provisions. The Company reviews those refund provisions to determine the likelihood of repayment. If the likelihood of repayment of the grant is determined to be remote, the grant is recognized as revenue. If the probability of repayment is determined to be more than remote, the Company records the amount of potential repayment of the grant as a liability, until such time that the grant requirements have been satisfied. Funds received in advance of the performance of the services are recorded as deferred revenue.

Research and Development Expenses

Research and development costs are charged to expense as costs are incurred. Research and development costs include salaries and benefits, other personnel costs, facilities costs, overhead costs, preclinical and clinical contract services, regulatory, manufacturing and other related costs. Up-front fees incurred in obtaining technology licenses are charged to research and development expense as incurred if the technology licensed has no alternative future use. Advance payments for goods or services for future research and development activities are deferred and expensed as the goods are delivered or the related services are performed.

The Company estimates research and development expenses for manufacturing activities, process development, preclinical studies and clinical trials based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage the activities on the Company’s behalf. In accruing these research and development expenses, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. These estimates are based on communications with the third party service providers and other information available to the Company at each balance sheet date. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. There have been no material changes from the Company’s original estimates in any of the periods presented.

Stock-based Compensation

The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (ASC 718). ASC 718 requires all stock-based payments to employees, including grants of employee stock options and restricted stock units, to be recognized in the consolidated statement of operations and comprehensive loss based on their fair values.

The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees with service-based vesting conditions is recognized on a straight-line basis based on the estimated grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to non-employees with service-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term, on a straight-line basis. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the estimated grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. Compensation expense related to awards to non-employees with performance-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable.

The Company estimates the fair value of its stock option awards to employees and directors using the Black-Scholes option-pricing model, which requires the input of and subjective assumptions, including (i) the fair value of the underlying common stock, (ii) the expected stock price volatility, (iii) the calculation of expected term of the award, (iv) the risk-free interest rate and (v) expected dividends. Due to the lack of company specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company will continue to apply this method of estimating expected volatility until a sufficient amount of historical information regarding the volatility of its own common stock price becomes available. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay dividends in the foreseeable future.

For the years ended December 31, 2016 and 2015, the Company was also required to estimate forfeitures of share-based awards at the time of grant and revise those estimates in subsequent periods if actual forfeitures differ from its estimates. For all periods through December 31, 2016, a forfeiture rate of zero was been used to calculate stock-based compensation expense due to the lack of historical information necessary to estimate forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were revised.

On January 1, 2017, the Company adopted ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. Under the new standard, the Company has elected not to estimate forfeitures of share-based awards and to account for forfeitures as they occur. The adoption of this standard required retrospective application, however, since the Company had previously assumed a forfeiture rate of zero on all share-based awards there was no impact to the consolidated financial statements as a result of the adoption of this standard.

The Company has granted stock options to non-employees as compensation for advisory services provided to the Company. Consistent with the guidance in FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees, the fair value of each non-employee stock option is estimated at the date of grant using the Black-Scholes option-pricing model with assumptions generally consistent with those used for employee stock options, with the exception of expected term, which is based on the contractual life.

Income Taxes

Income taxes are recorded in accordance with FASB ASC Topic 740, Income Taxes (ASC 740), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the consolidated financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances.

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2017 and 2016, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s consolidated statements of operations and comprehensive loss.

Net Loss Per Share

The Company computes net loss per share in conformity with the two-class method required for participating securities. The Company considers all series of convertible preferred stock outstanding prior to the IPO to be participating securities. The holders of convertible preferred stock outstanding prior to the IPO were entitled to receive preferential dividends in the event that a dividend was to be paid to the holders of common stock, and did not have a contractual obligation to share in the losses of the Company. As such, the Company’s net losses for the year ended December 31, 2015 were not allocated to these participating securities. In connection with the IPO, all outstanding shares of convertible preferred stock were automatically converted into shares of common stock.

Basic net loss per share is calculated by dividing net loss applicable to holders of common stock by the weighted-average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net loss per share until the contingency has been fully met. For purposes of the diluted net loss per share calculation, convertible preferred stock, outstanding stock options, outstanding restricted stock units and withholdings under the employee stock purchase plan are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive. Accordingly, basic and diluted net loss per share were the same for all periods presented.

Comprehensive Loss

The Company’s comprehensive loss includes its net loss as well as net unrealized gains and losses on available-for-sale securities, net of income tax effects and reclassification adjustments for realized gains and losses.

Recent Accounting Pronouncements

Recently adopted

In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, which is intended to simplify several aspects of the accounting for share-based payment awards including income tax consequences, classification of awards as either equity or liabilities and classification within the statement of cash flows. Additionally, the standard allows companies to elect not to estimate forfeitures of share-based awards for purposes of recognizing stock-based compensation expense, and account for forfeitures as they occur. The Company adopted this standard effective January 1, 2017, and upon adoption, has elected not to estimate forfeitures of share-based awards, which requires retrospective application in the consolidated financial statements. The Company has estimated a forfeiture rate of zero on all share-based awards granted since its inception and the adoption of this standard had no material impact on the Company’s tax provision or classification of the Company’s share-based awards. Accordingly, the application of this standard had no material impact on the Company’s financial position or results of operations.

Not yet adopted

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition. This ASU is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The ASU also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606), which deferred the effective date of the guidance under ASU 2014-09 by one year. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies various aspects of Topic 606, including the identification of performance obligations and the implementation of licensing guidance. In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which clarifies various additional aspects of Topic 606, including the assessment of collectability, presentation of sales taxes and other similar taxes collected from customers, the measurement date for transactions with non-cash consideration as well as transitional issues and other technical corrections regarding the adoption of new standards under Topic 606. In December 2016, the FASB issued ASU 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, which clarifies various additional narrow aspects of Topic 606. The standards are effective for annual and interim reporting periods beginning after December 15, 2017. Earlier application is permitted only as of annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. The Company has elected not to early adopt these standards. The Company believes that the adoption of Topic 606 will have a material impact on the Company’s revenue recognition policies and disclosures for license revenue and will result in the acceleration of revenue related to annual maintenance fees payable by licensees through the contractual terms of the agreements. Additionally, management believes that the accelerated recognition of revenue from maintenance fees expected to be received from licensees in future periods results in a significant financing component to the license, since such consideration is associated with performance obligations satisfied upon the delivery of the intellectual property license(s) at the onset of the agreements. As a result, the portion of the maintenance fees associated with the financing component will be recognized as interest income over the term of the license rather than license revenue. The changes to revenue recognition resulting from the adoption of Topic 606 will also impact the timing of recognition of sublicense fees payable by the Company to its licensors. The Company expects to utilize the modified retrospective method of application for the adoption of Topic 606. Under this method, the cumulative impact of the adoption through December 31, 2017 will be recorded through retained earnings on January 1, 2018, the adoption date. While the Company believes that the application of Topic 606 may have a material impact upon adoption, management has not yet finalized its analysis of the quantitative effects on the Company’s financial position or results of operations.

In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting, which clarifies when modification accounting should be applied for changes to terms or conditions of a share-based award. The standard is effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted, and is to be applied prospectively upon adoption. The Company does not believe the application of this standard will have a material impact on the Company’s financial position or results of operations.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The standard requires that the statement of cash flows explain the change during the period in the total of cash, cash equivalents and restricted cash. As a result, amounts generally described as restricted cash should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The standard is effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted, and is to be applied retrospectively to each period presented. The Company does not believe the application of this standard will have a material impact on the Company’s financial position or results of operations.

In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which amends the accounting for credit losses for most financial assets and certain other instruments. The standard requires that entities holding financial assets and net investment in leases that are not accounted for at fair value through net income be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The standard is effective for annual and interim periods beginning after December 15, 2019, with early adoption permitted for annual and interim periods beginning after December 15, 2018. The Company does not believe the application of this standard will have a material impact on the Company’s financial position or results of operations.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) which supersedes FASB ASC Topic 840, Leases (Topic 840) and provides principles for the recognition, measurement, presentation and disclosure of leases for both lessees and lessors. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. The standard is effective for annual and interim periods beginning after December 15, 2018, with early adoption permitted upon issuance. The Company is evaluating the application of this ASU, but has not yet determined the potential effects it may have on the Company’s consolidated financial statements.

In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which supersedes the current guidance to classify equity securities with readily determinable fair values into different categories and requires equity securities to be measured at fair value with changes in the fair value recognized through net income (loss). In February 2018, the FASB issued ASU 2018-03, Technical Corrections and Improvements to Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, that clarifies the guidance in ASU 2016-01. The standards are effective for annual and interim periods beginning after December 15, 2017. The Company does not believe the application of these standards will have a material impact on the Company’s financial position or results of operations.

Marketable Securities

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale securities (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2017
Corporate bonds	$	130,018	$	2	$	(282	)	$	129,738
	$	130,018	$	2	$	(282	)	$	129,738

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2016
Corporate bonds	$	133,424	$	82	$	(298	)	$	133,208
Common equity securities		300		618		—			918
	$	133,724	$	700	$	(298	)	$	134,126

Common equity securities reported as of December 31, 2016 consisted of shares of common stock of Audentes Therapeutics, Inc. (Audentes). The Company obtained these shares in connection with a license granted to Audentes in July 2013. Upon Audentes’ IPO in July 2016, the Company classified these shares as available-for-sale securities and recognized an unrealized gain of $0.6 million. Prior to Audentes’ IPO, the shares were not marketable and were accounted for by the Company as a cost method investment. The Company sold all of its shares of Audentes common stock during the year ended December 31, 2017.

As of December 31, 2017 and 2016, no available-for-sale securities had remaining maturities greater than three years.

The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. As of December 31, 2017 and 2016, the balance in the Company’s accumulated other comprehensive loss consisted solely of net unrealized gains and losses on available-for-sale securities, net of income tax effects and reclassification adjustments for realized gains and losses. During the years ended December 31, 2017, 2016 and 2015, the Company recognized net unrealized gains (losses) on available-for-sale securities of $(0.2) million, $1.1 million and $(0.7) million, respectively, and income tax expense of $0, $0.4 million and $0, respectively, in other comprehensive income (loss). The Company recognized net realized gains (losses) of $0.5 million, less than $(0.1) million and $0 on the sale or maturity of available-for-sale securities during the years ended December 31, 2017, 2016 and 2015, respectively, which were reclassified out of accumulated other comprehensive loss during the periods and are included in investment income in the consolidated statements of operations and comprehensive loss.

The aggregate fair value of securities held by the Company in an unrealized loss position for more than 12 months as of December 31, 2017 was $17.1 million. The aggregate unrealized loss of securities held by the Company in an unrealized loss position for more than 12 months as of December 31, 2017 was $0.1 million. The Company did not hold any securities in an unrealized loss position for more than 12 months as of December 31, 2016. The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of December 31, 2017 and 2016 was $109.2 million and $102.5 million, respectively. As of December 31, 2017, securities held by the Company which were in an unrealized loss position consisted of 46 investment grade corporate bond positions. The Company has the intent and ability to hold such securities until recovery and has determined that none of its investments were other-than-temporarily impaired as of December 31, 2017 or 2016.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

Financial instruments reported at fair value on a recurring basis include cash equivalents and marketable securities. Cash equivalents consist solely of money market mutual funds. Marketable securities consist of corporate debt securities as well as common equity securities as disclosed in Note 3. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands):

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2017
Money market mutual funds (cash equivalents)	$	—	$	46,646	$	—	$	46,646
Corporate bonds (marketable securities)		—		129,738		—		129,738
	$	—	$	176,384	$	—	$	176,384

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2016
Money market mutual funds (cash equivalents)	$	—	$	24,840	$	—	$	24,840
Corporate bonds (marketable securities)		—		133,208		—		133,208
Common equity securities (marketable securities)		918		—		—		918
	$	918	$	158,048	$	—	$	158,966

There were no transfers of financial instruments between levels of the fair value hierarchy during the years ended December 31, 2017 and 2016.

Management estimates that the carrying amounts of its accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments.

The Company has determined that it is not practicable to estimate the fair value of cost method investments. The Company has not identified any events or changes in circumstances that would have an adverse effect on the fair value of its cost method investments reported as of December 31, 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consists of the following (in thousands):

	December 31, 2017			December 31, 2016
Computer equipment and software	$	1,481		$	1,038
Lab equipment		8,561			2,780
Furniture and fixtures		1,384			1,213
Leasehold improvements		5,828			4,917
Total property and equipment		17,254			9,948
Accumulated depreciation and amortization		(3,277	)		(624	)
Property and equipment, net	$	13,977		$	9,324

During the years ended December 31, 2017, 2016 and 2015, the Company recorded depreciation expense of $2.7 million, $0.5 million and $0.1 million, respectively.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Lease Agreements

The Company recognizes rent expense on a straight-line basis over the term of its operating leases commencing on the date the Company takes possession of the leased property. Tenant improvement allowances which are considered to be lease incentives from the lessor are recorded as deferred rent and amortized as a reduction of rent expense over the term of the lease from the possession date.

In March 2015, the Company entered into a non-cancelable operating lease for office space at 9712 Medical Center Drive in Rockville, Maryland (the Medical Center Drive Lease). The lease term commenced in April 2015 and monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement.

In September 2015, November 2015 and July 2017, the Company amended the Medical Center Drive Lease to include additional office and laboratory space at 9714 Medical Center Drive, and ultimately extend the term of the lease to September 2021. The Company has options to extend the term of the Medical Center Drive Lease for up to six additional years. Under the amended lease, the Company has received a $0.4 million tenant improvement allowance from the landlord which will be deferred and amortized on a straight-line basis as a reduction of rent expense over the term of the lease.

In January 2016, the Company entered into a 7.5-year, non-cancelable operating lease for its corporate headquarters at 9600 Blackwell Road in Rockville, Maryland (the Blackwell Road Lease). The lease commenced in February 2016, and expires in September 2023. The Company has an option to extend the term of the lease for an additional five years. In November 2017, the Blackwell Road Lease was amended to include additional office space for the remainder of the lease term. Monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. The Company has received a $0.8 million tenant improvement allowance from the landlord which will be deferred and amortized on a straight-line basis as a reduction of rent expense over the term of lease.

In May 2016, the Company entered into a 51-month, non-cancelable operating lease for additional office space at 400 Madison Avenue in New York, New York. The lease commenced in July 2016, and expires in October 2020. Monthly payments under the lease began in October 2016 and escalate annually in accordance with the lease agreement. Under the terms of the lease agreement, the Company has provided the landlord with an irrevocable letter of credit of $0.2 million which the landlord may draw upon in the event of any uncured default by the Company under the terms of the lease. As of December 31, 2017 and 2016, the Company had recorded restricted cash of $0.2 million as collateral to the financial institution which issued the letter of credit.

As of December 31, 2017, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2018	$	2,172
2019		2,297
2020		2,310
2021		1,720
2022		621
Thereafter		479
Total minimum lease payments	$	9,599

Rent expense under all operating leases, including additional rent charges for utilities, parking, property management, operating expenses and real estate taxes for the years ended December 31, 2017, 2016 and 2015 was $1.8 million, $1.8 million and $0.4 million, respectively.

Licenses Granted to the Company

Licenses granted to the Company may require the Company to make future payments relating to sublicense fees, milestone fees for milestones achieved in the future and royalties on future sales of licensed products. Additionally, the Company may be responsible for the cost of the maintenance of the intellectual property as incurred by its licensors. Up-front fees to obtain licensed technology are included in research and development expenses and patent maintenance costs are included in general and administrative expenses in the consolidated statements of operations and comprehensive loss. Sublicense fees are based on a specified percentage of license fees earned by the Company and are included in licensing costs in the consolidated statements of operations and comprehensive loss. Milestone fees are included in licensing costs in the consolidated statements of operations and comprehensive loss if the underlying milestone is achieved by a licensee, or in research and development expense if the underlying milestone is achieved by the Company as a result of the development of its product candidates. Royalties on sales of licensed reagents for use in research and development are included in costs of reagent sales in the consolidated statements of operations and comprehensive loss. The Company has not commercialized any product candidates or paid any royalties under these agreements other than for the sales of licensed reagents.

The Trustees of the University of Pennsylvania. In February 2009, the Company entered into a license agreement, which has been amended from time to time, with The Trustees of the University of Pennsylvania (together with the University of Pennsylvania, Penn) for exclusive, worldwide rights to certain patents owned by Penn underlying the Company’s NAV Technology Platform. Under the terms of the agreement, in consideration for the license, the Company issued to Penn a 24.5% equity interest in the Company on a fully diluted basis after issuance. The Company is obligated to pay Penn royalties on net sales and sublicense fees, if any. Additionally, the Company is obligated to reimburse Penn for certain costs incurred related to the maintenance of the licensed patents.

In April 2016, the Company entered into an agreement with Penn whereby the Company will fund clinical trial activities performed by Penn for RGX-501, the Company’s product candidate for homozygous familial hypercholesterolemia (HoFH). In connection with the agreement, the Company amended its license from Penn to include exclusive license rights to data, results and other information generated in connection with the RGX-501 clinical trial.

Expenses incurred by the Company related to its license from Penn were as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Sublicense fees	$	207	$	430	$	703
Royalties on sales of reagents		4		11		9
Maintenance of licensed patents		233		133		154
	$	444	$	574	$	866

As of December 31, 2017 and 2016, the Company had accrued less than $0.1 million and $0.2 million, respectively, in expenses payable to Penn under the license agreement, which are included in accounts payable and accrued expenses on the Company’s consolidated balance sheets. Until September 30, 2015, the Company considered Penn to be a related party. For the nine months ended September 30, 2015, the Company incurred $0.4 million in expenses for sublicense fees, royalties on reagent sales and reimbursement of patent maintenance costs under its license from Penn. See Note 12 for further information on related party transactions with Penn.

GlaxoSmithKline LLC. In March 2009, the Company entered into a license agreement, which was amended in April 2009, with GlaxoSmithKline LLC (GSK) for exclusive, worldwide rights to certain patents underlying the Company’s NAV Technology Platform which are owned by Penn and exclusively licensed to GSK. Under the terms of the agreement, in consideration for the license the Company issued to GSK a 19.9% equity interest in the Company on a fully diluted basis after issuance. The Company is obligated to pay GSK royalties on net sales and sublicense fees, if any. Additionally, the Company is obligated to reimburse GSK for certain costs incurred and invoiced to the Company related to the maintenance of the licensed patents. The Company is also obligated to pay GSK up to $1.5 million upon the achievement of various milestones. From the inception of the agreement through December 31, 2017, the Company has incurred $0.5 million for milestones that have been achieved, or are deemed probable of achievement.

Expenses incurred by the Company related to its license from GSK were as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Sublicense fees	$	997	$	430	$	703
Royalties on sales of reagents		2		7		5
Milestone fees		500		—		—
Maintenance of licensed patents		509		597		573
	$	2,008	$	1,034	$	1,281

As of December 31, 2017 and 2016, the Company had accrued $0.3 million and $0.4 million, respectively, in expenses payable to GSK under the license agreement, which are included in accounts payable and accrued expenses on the Company’s consolidated balance sheets. Until September 30, 2015, the Company considered GSK to be a related party. For the nine months ended September 30, 2015, the Company incurred $0.7 million in expenses for sublicense fees, royalties on reagent sales and reimbursement of patent maintenance costs under its license from GSK. See Note 12 for further information on related party transactions with GSK.

Regents of the University of Minnesota. In November 2014, the Company entered into a license agreement, which was amended in November 2016, with Regents of the University of Minnesota (Minnesota), for an exclusive license under certain patent rights to commercialize products covered by the licensed patent rights in any country or territory in which a licensed patent has been issued and is unexpired, or a licensed patent application is pending. In consideration for the license, the Company paid an up-front fee, and reimbursed Minnesota for patent maintenance expenses, for a total of less than $0.1 million. Under the terms of the agreement, the Company is obligated to pay Minnesota annual maintenance fees of less than $0.1 million per year on each anniversary date of the agreement. Additionally, the Company is obligated to pay royalties on net sales and sublicense fees, if any, and up to $0.1 million per licensed product upon the achievement of various milestones. In November 2016, the license with Minnesota was amended to include additional patent rights. In consideration for the additional patent rights, the Company paid an up-front fee of less than $0.1 million.

From the inception of the agreement through December 31, 2017, the Company has incurred less than $0.1 million for milestones that have been achieved, or are deemed probable of achievement. The Company has not incurred any royalties or sublicense fees payable to Minnesota since the inception of the license agreement. During the years ended December 31, 2017, 2016 and 2015, the Company incurred $0.1 million, $0.1 million and less than $0.1 million, respectively, in total expenses under the license agreement with Minnesota. As of December 31, 2017 and 2016, the Company had accrued $0.1 million and less than $0.1 million, respectively, in expenses payable to Minnesota under the license agreement.

Guarantees and Indemnifications

In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. The Company’s potential exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of December 31, 2017 and 2016, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently has not recorded any related liabilities.

European Patent Office Proceeding

In June 2017, a third party filed an opposition with the European Patent Office challenging the validity of a European patent owned by Penn for the AAV8 vector, which the Company has exclusively licensed. This matter is in its early stages and the Company is unable to estimate the timing or outcome of this matter but intends to assist Penn in vigorously defending this patent. As of December 31, 2017, the Company has not recorded any liabilities related to this matter.

Capitalization

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Capitalization

7. Capitalization

As of December 31, 2017 and 2016, the authorized capital stock of the Company included 100,000,000 shares of common stock, par value $0.0001 per share, and 10,000,000 shares of preferred stock, par value $0.0001 per share. The Company’s restated certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares.

Issuance of Series C Preferred Stock

On January 13, 2015, the Company completed the sale and issuance of 4,631,774 shares of Series C convertible preferred stock (Series C Preferred Stock), par value $0.0001 per share, at a per share price of $6.477 for aggregate gross proceeds of $30.0 million, which included $26.0 million in cash proceeds, net of issuance costs of $0.2 million, and the conversion of $3.8 million of debt by FOXKISER LLP (FOXKISER).

Extinguishment of Preferred Stock

In connection with the issuance of the Series C Preferred Stock in January 2015, the rights, preferences and privileges of the Series A convertible preferred stock (Series A Preferred Stock) and Series B convertible preferred stock (Series B Preferred Stock) then outstanding were modified. More specifically, Series C Preferred Stock received preference in dividends and liquidation proceeds over Series A Preferred Stock and Series B Preferred Stock. Additionally, the dividend rights changed from cumulative dividend rights to non-cumulative dividend rights, and all accrued but unpaid cumulative dividends on the Series A Preferred Stock and Series B Preferred Stock as of January 13, 2015 were forfeited. As a result of this modification, the redemption value and liquidation preferences of Series A Preferred Stock and Series B Preferred Stock, which were previously equal to original issue price plus accrued but unpaid cumulative dividends, were reduced to original issue price plus non-cumulative dividends declared. Additionally, the redemption date of Series A Preferred Stock and Series B Preferred Stock was changed from October 30, 2018 to December 31, 2019.

The Company has accounted for the amendment to the rights, preferences and privileges of the Series A Preferred Stock and Series B Preferred Stock as an extinguishment of the original convertible preferred stock and issuance of new convertible preferred stock due to the significance of the modifications to the substantive contractual terms of the convertible preferred stock and the associated fundamental changes to the nature of the convertible preferred stock. Accordingly, upon extinguishment the Company recorded a loss of $1.3 million on the Series A Preferred Stock and a gain of $2.1 million on the Series B Preferred Stock within stockholders’ equity (deficit) equal to the difference between the fair value of the new shares of preferred stock issued and the carrying amount of the old shares of preferred stock extinguished. The Company allocated the entire net gain on extinguishment of convertible preferred stock of $0.8 million to additional paid-in capital. The net gain on extinguishment is reflected in the calculation of net loss available to common stockholders in accordance with FASB ASC Topic 260, Earnings per Share. The fair value of the Series A Preferred Stock and Series B Preferred Stock was determined using the option-pricing method (OPM) back-solve method on the per share price of Series C Preferred Stock to estimate aggregate equity value. The OPM was used to allocate equity value to the Series A Preferred Stock and Series B Preferred Stock using the Black-Scholes option-pricing model.

Issuance of Series D Preferred Stock

On May 15, 2015, the Company completed the sale and issuance of 7,366,849 shares of Series D convertible preferred stock (Series D Preferred Stock), par value $0.0001 per share, at a per share price of $9.5699 for proceeds of $68.0 million, net of issuance costs of $2.5 million.

Outstanding shares of Series A Preferred Stock, Series B Preferred Stock, Series C Preferred Stock and Series D Preferred Stock, were presented outside of stockholders’ equity (deficit) due to the rights, preferences and privileges of these securities, including redemption features. These securities were marked to their respective redemption values at each reporting date and the accretion of the securities was excluded from net loss applicable to common stockholders for purposes of calculating net loss per share.

Conversion of Convertible Preferred Stock upon IPO

Prior to the closing of the IPO on September 22, 2015, all of the outstanding shares of the Company’s convertible preferred stock were converted into 16,298,045 shares of its common stock. As of December 31, 2017 and 2016, the Company did not have any preferred stock outstanding.

The Company’s reserved shares of common stock for future issuance are as follows (in thousands):

	December 31, 2017		December 31, 2016
Reserved for issuance under equity incentive plans		7,430		6,887
Reserved for issuance under employee stock purchase plan		206		254
		7,636		7,141

Significant Agreements

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Significant Agreements

8. Significant Agreements

See Note 6 for licenses granted to the Company.

Licenses Granted by the Company

The Company has granted a number of intellectual property licenses to other biotechnology and pharmaceutical companies. The terms of the licenses vary, however licenses may be exclusive or non-exclusive and may be sublicensable by the licensee. Licenses may grant intellectual property rights for purposes of internal and preclinical research and development only, or may include the rights, or options to obtain future rights, to commercialize drug therapies for specific diseases using the Company’s NAV Technology Platform. License agreements generally have a term at least equal to the life of the underlying patents and are terminable only at the option of the licensee. License agreements may require licensees to pay non-refundable up-front fees, option fees and annual maintenance fees. Additional contingent consideration under the licenses may include sublicense fees, milestone fees and royalties on net sales of commercialized products. Sublicense fees vary by license and range from a mid-single digit percentage to a low-double digit percentage of license fees received by licensees as a result of sublicenses. Royalties on net sales of commercialized products vary by license and range from a mid-single digit percentage to a low-teen percentage of net sales by licensees.

Milestone fees are payable to the Company upon the achievement of specific clinical, regulatory and commercial developments by licensees. As of December 31, 2017, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, could result in aggregate milestone fees payable to the Company of up to $27.8 million upon the commencement of various stages of clinical trials, $48.5 million upon the submission of regulatory approval filings, $110.5 million upon the approval of commercial products by regulatory agencies and $92.0 million upon the achievement of specified sales targets for licensed products. The achievement of milestones by licensees is highly dependent on the successful development and commercialization of licensed products and it is at least reasonably possible that some or all of the milestone fees will not be realized by the Company.

License revenue consists of the following (in thousands):

	Years Ended December 31,
	2017		2016		2015
Up-front fees and option fees for commercial licenses	$	6,420	$	3,050	$	5,900
Maintenance fees for commercial licenses		920		910		635
Milestone fees		1,850		—		250
Research and other license revenue		1,195		343		240
	$	10,385	$	4,303	$	7,025

AveXis, Inc. License to Treat Spinal Muscular Atrophy, as Amended. In January 2018, the Company entered into an amendment (the January 2018 Amendment) to its March 2014 license agreement (the March 2014 License) with AveXis, Inc. (AveXis). Under the January 2018 Amendment, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to any recombinant AAV vector in the Company’s intellectual property portfolio during the term of the license agreement for the treatment of spinal muscular atrophy (SMA) in humans by in vivo gene therapy. Under the original March 2014 License, the Company had granted AveXis a license relating only to the NAV AAV9 vector.

Additionally, the January 2018 Amendment modifies the terms and conditions of the March 2014 License relating to assignment. Under the amended assignment provision, AveXis is permitted to transfer the March 2014 License, as amended, without the Company’s consent in connection with a change of control of AveXis, subject to the transferee or successor agreeing in writing to be bound by the terms of the March 2014 License, as amended, and the payment to the Company of certain fees due upon such change of control, as described below. Under the original March 2014 License, any assignment by AveXis without the Company’s prior written consent had been prohibited. The January 2018 Amendment also provides that, solely upon the written request of AveXis, the Company may, in its sole discretion after receipt of such request, provide certain collaboration services to AveXis regarding the development and commercialization of gene therapy product candidates for the treatment of SMA.

Pursuant to the January 2018 Amendment, in consideration for the additional rights granted thereunder and in addition to the payments and royalties owed under the original March 2014 License, AveXis paid to the Company a fee of $80.0 million upon entry into the January 2018 Amendment. In addition, AveXis will pay to the Company (i) $30.0 million on the first anniversary of the effective date of the January 2018 Amendment, (ii) $30.0 million on the second anniversary of the effective date of the January 2018 Amendment and (iii) potential commercial milestone payments of up to $120.0 million. In the event of a change of control of AveXis, to the extent that any fee described in (i) or (ii) above, or the first $40.0 million of milestone payments described in (iii) above, has not yet been paid to the Company, AveXis will be obliged to pay any such unpaid fee to the Company upon the change of control. Additionally, for any product developed for the treatment of SMA using the NAV AAV9 vector, the Company will continue to receive mid-single to low double-digit royalties on net sales as defined in the original March 2014 License, and for any product developed for the treatment of SMA using a NAV vector other than NAV AAV9, the Company will receive a low double-digit royalty on net sales.

During the years ended December 31, 2017, 2016 and 2015, the Company recognized license revenue of $1.1 million, $0.1 million and $0.3 million, respectively, under the March 2014 License. The Company did not recognize any revenue in 2017 related to the amendment of the license in January 2018.

AveXis, Inc. License to Treat Rett Syndrome and Amyotrophic Lateral Sclerosis. In June 2017, the Company entered into an exclusive license agreement (the June 2017 License) with AveXis for the development and commercialization of products to treat Rett Syndrome and amyotrophic lateral sclerosis (ALS) caused by mutations in the gene that produces the copper zinc superoxide dismutase 1 (SOD1) enzyme using AAV9. Under the license agreement, the Company granted AveXis an exclusive, sublicensable worldwide license under the licensed intellectual property to make, have made, use, import, sell and offer for sale licensed products in the treatment of Rett Syndrome and ALS caused by mutations in the SOD1 gene using AAV9. In consideration for the license, AveXis paid the Company an up-front fee of $6.0 million, and is required to pay annual maintenance fees, milestone fees of up to $36.0 million, a low double digit royalty percentage on net sales of licensed products, subject to reduction in specified circumstances, and a lower mid-double digit percentage of any sublicense fees AveXis receives from sublicensees for the licensed intellectual property rights. The Company recognized license revenue of $6.0 million under the June 2017 License during the year ended December 31, 2017.

Stock-based Compensation

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

9. Stock-based Compensation

In September 2014, the Board of Directors adopted the 2014 Stock Plan (2014 Plan). In June 2015, the Board of Directors adopted the 2015 Equity Incentive Plan (2015 Plan), which became effective on September 16, 2015, the date on which the registration statement for the IPO was declared effective. The 2015 Plan replaced the 2014 Plan, and as of the effective date of the 2015 Plan, no further awards may be issued under the 2014 Plan. Any options or awards outstanding under the 2014 Plan as of the effective date of the 2015 Plan remained outstanding and effective. The number of authorized shares under the 2015 Plan automatically increases annually on January 1, beginning January 1, 2016, by the lesser of (i) 4% of the total number of shares of common stock outstanding on December 31 of the prior year, or (ii) a number of common shares determined by the Board of Directors. In January 2016 and 2017, the Board of Directors authorized an additional 1,052,538 and 1,059,065 shares, respectively, to be issued under the 2015 Plan. As of December 31, 2017, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Plan was 8,236,603 of which 1,922,892 remain available for future grants under the 2015 Plan. In January 2018, the Board of Directors authorized an additional 1,251,810 shares to be issued under the 2015 Plan.

The 2014 Plan and 2015 Plan provide for the issuance of stock options, stock appreciation rights, restricted and unrestricted stock and unit awards, and performance cash awards to employees, members of the Board of Directors and consultants of the Company. Since the inception of the plans, the Company has issued only stock options and restricted stock units under the plans. Stock options under the 2014 Plan and 2015 Plan generally expire ten years following the date of grant. Options typically vest over a four-year period, but vesting provisions can vary by award based on the discretion of the Board of Directors. Certain stock option awards granted by the Company include performance conditions that must be achieved in order for vesting to occur. Stock options under the 2014 Plan and 2015 Plan carry an exercise price at least equal to the estimated fair value of the Company’s common stock on the date of grant. Restricted stock units vest in accordance with the underlying award agreements and, upon vesting, are settled in common stock of the Company.

Shares of common stock underlying awards previously issued under the 2014 Plan and 2015 Plan which are reacquired by the Company, withheld by the Company in payment of the purchase price, exercise price or withholding taxes, expired, cancelled due to forfeiture or otherwise terminated other than by exercise or settlement, are added to the number of shares of common stock available for issuance under the 2015 Plan. Shares available for issuance under the 2015 Plan may be either authorized but unissued shares of the Company’s common stock or common stock reacquired by the Company and held in treasury. The 2015 Plan expires in June 2025, ten years from the date it was adopted by the Board of Directors, unless earlier terminated.

Stock-based Compensation Expense

The Company’s stock-based compensation expense by award type is as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Stock options	$	10,031	$	6,950	$	2,921
Restricted stock units		275		23		—
Employee stock purchase plan		299		58		—
	$	10,605	$	7,031	$	2,921

As of December 31, 2017, the Company had $22.9 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.5 years.

The Company has recorded aggregate stock-based compensation expense in the consolidated statement of operations and comprehensive loss as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	5,128	$	2,743	$	1,760
General and administrative		5,477		4,288		1,161
	$	10,605	$	7,031	$	2,921

Stock Options

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2016		5,118		$	7.77		8.5	$	57,400
Granted		1,155		$	20.44
Exercised		(516	)	$	4.83
Cancelled or forfeited		(289	)	$	16.59
Outstanding at December 31, 2017		5,468		$	10.25		7.9	$	125,738
Exercisable at December 31, 2017		2,979		$	6.24		7.4	$	80,482
Vested and expected to vest at December 31, 2017		5,468		$	10.25		7.9	$	125,738

(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

The weighted-average grant date fair value per share of options granted during the years ended December 31, 2017, 2016 and 2015 was $13.87, $8.37 and $7.08, respectively. During the years ended December 31, 2017, 2016 and 2015, the total number of stock options exercised was 515,916, 163,158 and 125,563, respectively, resulting in total proceeds of $2.5 million, $0.2 million and $0.1 million, respectively. The total intrinsic value of options exercised during the years ended December 31, 2017, 2016 and 2015 was $11.1 million, $1.7 million and $0.5 million, respectively.

Valuation of Common Stock. Prior to the Company’s IPO, the Company estimated the fair value of its common stock underlying stock option awards at the grant date of the award. Valuation estimates were prepared by management in accordance with the framework of the American Institute of Certified Public Accountants Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation, as well as through independent third-party valuations, and were approved by the Company’s Board of Directors. Valuation estimates were based on a variety of factors including the Company’s financial position and historical financial performance, the Company’s stage of development, the valuations of comparable publicly traded companies, marketplace and macro-economic factors, the illiquid nature of the common stock, arm’s-length sales of the Company’s preferred equity securities, the effect of the rights, preferences and privileges of preferred shareholders and the prospects of a liquidity event, among others. After the completion of the Company’s IPO in September 2015, the fair value of the common stock is based on the closing price of the common stock on the date of grant.

Stock Options Granted to Employees. For the years ended December 31, 2017, 2016 and 2015, the Company recorded $10.1 million, $6.4 million and $1.4 million, respectively, of stock-based compensation expense related to stock options granted to employees and directors. The fair value of options granted to employees and directors was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2017			2016			2015
Expected volatility		77	%		75	%		68	%
Expected term (in years)		6.2			6.2			6.1
Risk-free interest rate		2.1	%		1.5	%		1.7	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Stock Options Granted to Non-employees. Stock-based compensation expense related to stock options granted to non-employees is recognized as the stock options are earned. The Company believes that the estimated fair value of the stock options is more readily measurable than the fair value of the services rendered. For the years ended December 31, 2017, 2016 and 2015, the Company recorded $0.5 million, $0.6 million and $1.5 million, respectively, of stock-based compensation expense related to stock options granted to non-employees. The fair value of options granted to non-employees was estimated at each measurement date using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2017			2016			2015
Expected volatility		77	%		83	%		75	%
Expected term (in years)		7.7			8.5			9.5
Risk-free interest rate		2.2	%		1.7	%		2.1	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Restricted Stock Units

The following table summarize restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

			Weighted-
			average
			Grant Date
	Shares		Fair Value
Unvested balance at December 31, 2016		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at December 31, 2017		40	$	20.90

Employee Stock Purchase Plan

In June 2015, the Company’s Board of Directors adopted the 2015 ESPP, which became effective on September 16, 2015, the date on which the registration statement for the IPO was declared effective. The 2015 ESPP authorizes the initial issuance of up to a total of 254,000 shares of the Company’s common stock to participating employees. The number of shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year, commencing in 2016, by a number equal to the lesser of (i) 1% of the shares of common stock outstanding on the last business day of the prior fiscal year; or (ii) the number of shares determined by the Company’s Board of Directors. Unless otherwise determined by the administrator of the 2015 ESPP, two offering periods of six months’ duration will begin each year on January 1 and July 1. The Board of Directors did not authorize any further shares to be reserved for issuance under the 2015 ESPP in January 2016 or 2017. As of December 31, 2017, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 206,280 remain available for future issuance. During the year ended December 31, 2017, 47,720 shares of common stock were issued under the 2015 ESPP. There were no shares of common stock issued under the ESPP during the years ended December 31, 2016 and 2015.

Retirement Plan	12 Months Ended
Retirement Plan	Dec. 31, 2017
Postemployment Benefits [Abstract]
Retirement Plan	10. Retirement Plan In February 2015, the Company established a defined-contribution retirement plan under Section 401(k) of the Internal Revenue Code (the 401(k) Plan). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation. The Company matches employee deferrals up to 5.75% of eligible compensation. For the years ended December 31, 2017, 2016 and 2015, the Company contributed $1.0 million, $0.6 million and $0.2 million, respectively, in matching contributions to the 401(k) Plan. Prior to the establishment of the 401(k) Plan in February 2015, the Company did not sponsor any retirement plans.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

The substantial majority of the Company’s income and loss is generated in the U.S. and attributable to the U.S. tax jurisdiction. There was no foreign income tax expense (benefit) for the years ended December 31, 2017, 2016 and 2015.

The Tax Cuts and Jobs Act of 2017 (the TCJA) was signed into law in December 2017, and has resulted in significant changes to the U.S. corporate income tax system. The TCJA, among other things, contains significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21%, elimination, reduction or limitation of certain domestic deductions and credits, limitation of the deduction for net operating losses (NOLs) to 80% of current year taxable income, elimination of NOL carrybacks, one-time taxation of offshore earnings at reduced rates regardless of whether they are repatriated, elimination of U.S. tax on foreign earnings (subject to certain significant exceptions), immediate deductions for certain new investments instead of deductions for depreciation expense over time, and modification or repeal of many business deductions and credits, including the orphan drug tax credit. The changes resulting from the TCJA are effective beginning in 2018; however, the impact to the Company’s deferred tax assets and liabilities as a result of these changes has been accounted for as of December 31, 2017 on a provisional basis.

In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118), which allows the Company to record provisional amounts for the effects of the TCJA in the period it was enacted for a measurement period not extend beyond one year of the enactment date. In accordance with SAB 118, the Company has determined that the impact of the TCJA to its deferred tax assets and liabilities and valuation allowance as of December 31, 2017 is a reasonable estimate and provisional amount. The final impact of the TCJA may differ from this provisional amount due to changes in the Company’s estimates and the issuance of additional regulatory or other guidance. The Company expects to complete its assessment of the final impact of the TCJA within the required measurement period under SAB 118.

A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate of 34% to income tax expense (benefit) as reflected in the financial statements is as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
Federal income tax benefit at statutory rate	$	(24,876	)	$	(21,556	)	$	(7,756	)
State income tax benefit, net of federal benefit		(7,756	)		(1,617	)		(1,511	)
Step-up in assets upon conversion to C-corporation		—			—			(709	)
Research and development credits		(6,297	)		(7,275	)		—
Stock-based compensation expense for incentive stock options and employee stock purchase plan		(3,012	)		1,805			283
Other non-deductible expenses and reconciling items		(26	)		170			129
Change in corporate tax rates		16,598			42			111
Change in valuation allowance		25,369			27,996			9,453
Total income tax expense (benefit)	$	—		$	(435	)	$	—

The significant components of the Company’s net deferred tax assets are as follows (in thousands):

	December 31, 2017			December 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$	37,658		$	25,625
Research and development tax credits		21,137			10,890
Step-up in assets upon conversion to C-corporation		721			953
Stock-based compensation expense for non-qualified stock options and restricted stock units		4,263			1,401
Unrealized losses on marketable securities		89			—
Deferred rent		458			258
Accruals and other		1,540			1,132
Total deferred tax assets before valuation allowance		65,866			40,259
Valuation allowance		(65,669	)		(40,084	)
Total deferred tax assets		197			175
Deferred tax liabilities:
Depreciation		(197	)		(19	)
Unrealized gains on marketable securities		—			(156	)
Total deferred tax liabilities		(197	)		(175	)
Net deferred tax assets	$	—		$	—

The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for its net deferred tax assets as of December 31, 2017 and 2016. The valuation allowance increased approximately $25.6 million and $28.0 million during the years ended December 31, 2017 and 2016, respectively, due primarily to federal net operating losses and research and development credits generated during the periods. The net increase in the valuation allowance during the year ended December 31, 2016 includes a partially offsetting decrease of $0.4 million for the income tax effects of gains on available-for-sale securities recognized in other comprehensive income for the period.

As of December 31, 2017 and 2016, the Company had U.S. federal NOL carryforwards of approximately $126.7 million and $66.4 million, respectively, which may be available to offset future income tax liabilities and expire at various dates beginning in 2034. As of December 31, 2017 and 2016, the Company also had U.S. state NOL carryforwards of approximately $167.8 million and $55.0 million, respectively, which may be available to offset future income tax liabilities and expire at various dates beginning in 2035.

As of December 31, 2017, the Company had federal research and development and orphan drug tax credit carryforwards of approximately $21.3 million which may be available to reduce future tax liabilities and expire at various dates beginning in 2035. The Company has conducted a formal study to document the qualified activities and expenses used to calculate the credits through the year ended December 31, 2016. The remaining portion of the credits as of December 31, 2017 are subject to future studies which may result in an adjustment to the Company’s credit carryforwards. The calculation of these credits requires assumptions to be made by management to estimate qualified research expenses. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740, and accordingly, recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. As of December 31, 2017, the Company has unrecognized tax benefits of $0.1 million which have been reserved against the credit carryforwards as uncertain tax positions. No reserve for uncertain tax positions has been placed against qualified expenses for which a study has not been conducted. However, a full valuation allowance has been provided against the net credit carryforwards and, if an adjustment is required upon the completion of the study, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carryforwards and the valuation allowance.

Under the provisions of the Internal Revenue Code, the NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception which may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code, or could result in a change in control in the future.

The Company files income tax returns in the United States at the federal level and in states in which the Company conducts business activities. The federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2013 onward. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent utilized in a future period.

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions The Trustees of the University of Pennsylvania Penn received an equity interest in the Company as consideration for the intellectual property license granted to the Company in February 2009, and prior to September 30, 2015, the Company considered Penn to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from Penn. In addition to the license agreement, Penn also provides manufacturing and research and development services to the Company. During the nine months ended September 30, 2015, the Company incurred $0.1 million in services from Penn for the manufacturing of reagents for sale by the Company as well as $5.1 million in research and development services provided by Penn. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers Penn to be a related party subsequent to the period ended September 30, 2015. GlaxoSmithKline LLC GSK received an equity interest in the Company as consideration for the intellectual property license granted to the Company in March 2009, and prior to September 30, 2015, the Company considered GSK to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from GSK. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers GSK to be a related party subsequent to the period ended September 30, 2015. Dimension Therapeutics, Inc. The Company and a number of its members, directors and management received common stock of Dimension Therapeutics, Inc. (Dimension) as consideration for a license granted to Dimension in October 2013. Additionally, at the date of the license, three of the Company’s board members also served on the board of directors of Dimension. As a result, prior to September 30, 2015, the Company considered Dimension to be a related party. As of September 30, 2015, the Company no longer had any overlapping board members with Dimension. Additionally, the Company’s directors and management do not have significant influence over Dimension. Accordingly, the Company no longer considers Dimension to be a related party subsequent to the period ended September 30, 2015. The Company granted various commercial licenses, and options to obtain future commercial licenses, to Dimension prior to September 30, 2015. During the nine months ended September 30, 2015, the Company recognized license revenue of $2.0 million from commercial licenses granted to Dimension, which is included in license revenue from related party in the consolidated statements of operations and comprehensive loss. Revenue earned and recognized from Dimension subsequent to September 30, 2015 is no longer included in revenue from related party in the consolidated statements of operations and comprehensive loss. During 2014, the Company received $0.2 million from Dimension for the purchase of materials owned by the Company and used in the Company’s manufacturing process for research and development and clinical trials. The $0.2 million was recognized as a gain on disposal of the material in the periods in which the material was delivered to Dimension. During the nine months ended September 30, 2015, the Company recognized gains of less than $0.1 million for the delivery of the material to Dimension. FOXKISER LLP The Company was party to a services agreement, as amended from time to time, with FOXKISER, an affiliate of certain stockholders of the Company and an affiliate of one current and one former member of the Board of Directors, which was terminated in January 2015. Under the agreement, the Company paid a fixed monthly fee plus an additional support fee, as determined by FOXKISER on a monthly basis, as consideration for office facilities, equipment, supplies, general management, accounting, financial management, human resources, legal, secretarial, regulatory compliance and other services provided to the Company. The amounts outstanding to FOXKISER under the services agreement in excess of 30 days from their due date accrued interest at 1.5% per month, compounding monthly. The Company allocated the service fees from FOXKISER between research and development and general and administrative expense. For the year ended December 31, 2015, the Company incurred costs under the arrangement of $0.2 million which was allocated to research and development expense and $0.2 million which was allocated to general and administrative expense. Prior to 2015, the Company received cash proceeds and services from FOXKISER from time to time in exchange for various promissory notes and other debt instruments. Pursuant to the terms of the debt instruments, at the option of FOXKISER, the debts may be fully settled in shares of preferred equity securities of the Company. Interest expense to FOXKISER under these debt instruments for the year ended December 31, 2015 was less than $0.1 million. In January 2015, in conjunction with the Company’s issuance of Series C Preferred Stock, FOXKISER elected its share settlement options and converted $3.8 million of the outstanding debt payable by the Company into 585,578 shares of Series C Preferred Stock at a per share price of $6.477. The previously outstanding debt instruments were settled in full upon the conversion. In January 2015, the Company entered into an operating lease with FOXKISER for office space in Washington, D.C. The lease agreement, which had a month-to-month term, required monthly payments of less than $0.1 million. The lease was terminated in April 2015. Total rent expense to FOXKISER during the term of the lease was $0.1 million. Effective January 2016, the Company entered into a Professional Services Agreement with FOXKISER, pursuant to which the Company pays a fixed monthly fee in consideration for certain strategic planning, development and regulatory services provided by FOXKISER. The agreement expired in December 2016, and effective January 2017 the Company entered into a new Professional Services Agreement with FOXKISER which expired in December 2017. Effective January 2018, the Company entered into a new Professional Services Agreement, which has a term of one year and is terminable by either party, at any time, upon 60 days’ prior written notice to the other party. Costs incurred under the Professional Services Agreements with FOXKISER for the years ended December 31, 2017 and 2016 were $1.5 million and $1.0 million, respectively, and are recorded as research and development expenses in the consolidated statements of operations and comprehensive loss. Scientific Founder and Special Advisor In September 2014, the Company entered into an advisory agreement, as amended, with James M. Wilson, M.D., Ph.D., the Company’s Scientific Founder and Special Advisor (formerly the Company’s Chief Scientific Advisor), who is also the Chairman of the Company’s Scientific Advisory Board. The agreement required a fixed monthly payment in consideration for scientific advisory services and expired in March 2017. Dr. Wilson will continue to provide services at no cost to the Company pursuant to a new advisory agreement entered into in March 2017. During the years ended December 31, 2017, 2016 and 2015, the Company incurred advisory fees of $0.1 million, $0.3 million and $0.2 million, respectively, under the agreements. Additionally, in 2014 and 2015 the Company granted options to purchase a total of 211,600 shares of common stock to Dr. Wilson as compensation for the advisory services to be provided to the Company, which vest partially upon the completion of service conditions and partially upon the achievement of specified performance conditions as set forth in the award agreements.

Net Loss Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Share

13. Net Loss Per Share

The following potentially dilutive securities outstanding at the end of the period were excluded from the computations of diluted weighted-average shares outstanding for the periods indicated as they would be anti-dilutive (in thousands):

	Years Ended December 31,
	2017		2016		2015
Stock options issued and outstanding		5,468		5,118		3,684
Unvested restricted stock units outstanding		40		40		—
Employee stock purchase plan		20		22		—
		5,528		5,180		3,684

Amounts in the table above reflect the common stock equivalents of the noted instruments.

Supplemental Disclosures

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Supplemental Disclosures

14. Supplemental Disclosures

Accrued expenses and other current liabilities consists of the following (in thousands):

	December 31, 2017		December 31, 2016
Accrued personnel costs	$	5,789	$	3,338
Accrued external general and administrative expenses		1,078		672
Accrued external research and development expenses		2,072		3,005
Accrued purchases of property and equipment		430		844
Other accrued expenses and current liabilities		236		267
	$	9,605	$	8,126

Selected Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Information (Unaudited)

15. Selected Quarterly Financial Information (Unaudited)

The following table contains quarterly financial information for the years ended December 31, 2017 and 2016. Management believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. Amounts in the following table are in thousands, except per share data.

	Quarters Ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Total revenues	$	455		$	6,562		$	1,336		$	2,040
Research and development expense	$	16,619		$	13,917		$	12,518		$	14,170
General and administrative expense	$	6,622		$	6,355		$	9,444		$	4,808
Total operating expenses	$	23,377		$	21,618		$	22,645		$	18,638
Net loss	$	(21,993	)	$	(14,473	)	$	(20,706	)	$	(15,997	)
Basic and diluted net loss per common share	$	(0.82	)	$	(0.47	)	$	(0.67	)	$	(0.51	)

	Quarters Ended
	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
Total revenues	$	393		$	2,375		$	125		$	1,696
Research and development expense	$	6,183		$	10,680		$	12,560		$	16,059
General and administrative expense	$	5,479		$	6,169		$	6,200		$	5,742
Total operating expenses	$	11,644		$	17,327		$	18,793		$	22,165
Net loss	$	(10,768	)	$	(14,437	)	$	(18,154	)	$	(19,608	)
Basic and diluted net loss per common share	$	(0.41	)	$	(0.55	)	$	(0.69	)	$	(0.74	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Basis of Presentation and Principles of Consolidation

Foreign Currency Transactions

Use of Estimates

Reclassifications

Segment and Geographical Information

Cash and Cash Equivalents

Restricted Cash

Restricted cash includes money market mutual funds used to collateralize an irrevocable letter of credit as required by the Company’s lease agreement for its office space in New York, New York.

Marketable Securities

Concentrations of Credit Risk and Off-balance Sheet Risk

The following table summarizes those customers who represented at least 10% of revenue or accounts receivable for the periods presented:

Revenue

Accounts Receivable

Years Ended December 31,

December 31,

2017

2016

2015

2017

2016

Customer A

Customer B

Customer C⁽¹⁾

Customer D

Customer E

Customer F

Customer G

Customer H

Customer I

*	Represented less than 10%

(1)	Represents a related party prior to September 30, 2015

Accounts Receivable

Property and Equipment


Computer equipment and software			3 years
Lab equipment			5 years
Furniture and fixtures			5 years
Leasehold improvements			Shorter of lease term or estimated useful life

Impairment of Long-lived Assets

Cost Method Investments

Fair Value of Financial Instruments

•

Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

•

Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

•

Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

Revenue Recognition

•

Persuasive evidence of an arrangement exists;

•

Delivery has occurred or services have been rendered;

•

The seller’s price to the buyer is fixed or determinable; and

•

Collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s balance sheets.

Please refer to Note 12 for information regarding license revenue from related party.

Research and Development Expense

Research and Development Expenses

Share-based Compensation

Stock-based Compensation

Income Taxes

Net Loss Per Share

Comprehensive Loss

Recent Accounting Pronouncements

Recently adopted

Not yet adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Schedule of Revenue or Accounts Receivable by Major Customers

The following table summarizes those customers who represented at least 10% of revenue or accounts receivable for the periods presented:

Revenue

Accounts Receivable

Years Ended December 31,

December 31,

2017

2016

2015

2017

2016

Customer A

Customer B

Customer C⁽¹⁾

Customer D

Customer E

Customer F

Customer G

Customer H

Customer I

*	Represented less than 10%

(1)	Represents a related party prior to September 30, 2015

Summary of Estimated Useful Lives of Assets

Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:


Computer equipment and software			3 years
Lab equipment			5 years
Furniture and fixtures			5 years
Leasehold improvements			Shorter of lease term or estimated useful life

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Summary of Company Marketable Securities

The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale securities (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2017
Corporate bonds	$	130,018	$	2	$	(282	)	$	129,738
	$	130,018	$	2	$	(282	)	$	129,738

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2016
Corporate bonds	$	133,424	$	82	$	(298	)	$	133,208
Common equity securities		300		618		—			918
	$	133,724	$	700	$	(298	)	$	134,126

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair Value of Cash Equivalents and Marketable Securities

The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands):

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2017
Money market mutual funds (cash equivalents)	$	—	$	46,646	$	—	$	46,646
Corporate bonds (marketable securities)		—		129,738		—		129,738
	$	—	$	176,384	$	—	$	176,384

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2016
Money market mutual funds (cash equivalents)	$	—	$	24,840	$	—	$	24,840
Corporate bonds (marketable securities)		—		133,208		—		133,208
Common equity securities (marketable securities)		918		—		—		918
	$	918	$	158,048	$	—	$	158,966

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment, net consists of the following (in thousands):

	December 31, 2017			December 31, 2016
Computer equipment and software	$	1,481		$	1,038
Lab equipment		8,561			2,780
Furniture and fixtures		1,384			1,213
Leasehold improvements		5,828			4,917
Total property and equipment		17,254			9,948
Accumulated depreciation and amortization		(3,277	)		(624	)
Property and equipment, net	$	13,977		$	9,324

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases

As of December 31, 2017, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2018	$	2,172
2019		2,297
2020		2,310
2021		1,720
2022		621
Thereafter		479
Total minimum lease payments	$	9,599

The Trustees of the University of Pennsylvania [Member]

Summary of Expenses Incurred by Company

Expenses incurred by the Company related to its license from Penn were as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Sublicense fees	$	207	$	430	$	703
Royalties on sales of reagents		4		11		9
Maintenance of licensed patents		233		133		154
	$	444	$	574	$	866

GlaxoSmithKline LLC [Member]

Summary of Expenses Incurred by Company

Expenses incurred by the Company related to its license from GSK were as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Sublicense fees	$	997	$	430	$	703
Royalties on sales of reagents		2		7		5
Milestone fees		500		—		—
Maintenance of licensed patents		509		597		573
	$	2,008	$	1,034	$	1,281

Capitalization (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Schedule of Reserved Shares of Common Stock for Future Issuance

The Company’s reserved shares of common stock for future issuance are as follows (in thousands):

	December 31, 2017		December 31, 2016
Reserved for issuance under equity incentive plans		7,430		6,887
Reserved for issuance under employee stock purchase plan		206		254
		7,636		7,141

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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+ Details

Significant Agreements (Tables)

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of License Revenue

License revenue consists of the following (in thousands):

	Years Ended December 31,
	2017		2016		2015
Up-front fees and option fees for commercial licenses	$	6,420	$	3,050	$	5,900
Maintenance fees for commercial licenses		920		910		635
Milestone fees		1,850		—		250
Research and other license revenue		1,195		343		240
	$	10,385	$	4,303	$	7,025

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock-Based Compensation Expense by Award Type

The Company’s stock-based compensation expense by award type is as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Stock options	$	10,031	$	6,950	$	2,921
Restricted stock units		275		23		—
Employee stock purchase plan		299		58		—
	$	10,605	$	7,031	$	2,921

Stock-Based Compensation Expense

The Company has recorded aggregate stock-based compensation expense in the consolidated statement of operations and comprehensive loss as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	5,128	$	2,743	$	1,760
General and administrative		5,477		4,288		1,161
	$	10,605	$	7,031	$	2,921

Schedule of Fair Value of Options Granted to Employees and Directors Valuation Assumptions

The fair value of options granted to employees and directors was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2017			2016			2015
Expected volatility		77	%		75	%		68	%
Expected term (in years)		6.2			6.2			6.1
Risk-free interest rate		2.1	%		1.5	%		1.7	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Unvested RSUs Activity Under 2015 Plan

The following table summarize restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

			Weighted-
			average
			Grant Date
	Shares		Fair Value
Unvested balance at December 31, 2016		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at December 31, 2017		40	$	20.90

Non Employee Stock Option [Member]

Schedule of Non-Employees Stock Purchase Plan Valuation Assumptions

The fair value of options granted to non-employees was estimated at each measurement date using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2017			2016			2015
Expected volatility		77	%		83	%		75	%
Expected term (in years)		7.7			8.5			9.5
Risk-free interest rate		2.2	%		1.7	%		2.1	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

2014 and 2015 Equity Incentive Plan [Member]

Summary of Stock Option Activity

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2016		5,118		$	7.77		8.5	$	57,400
Granted		1,155		$	20.44
Exercised		(516	)	$	4.83
Cancelled or forfeited		(289	)	$	16.59
Outstanding at December 31, 2017		5,468		$	10.25		7.9	$	125,738
Exercisable at December 31, 2017		2,979		$	6.24		7.4	$	80,482
Vested and expected to vest at December 31, 2017		5,468		$	10.25		7.9	$	125,738

(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Effective Income Tax Expense (Benefit)

	Years Ended December 31,
	2017			2016			2015
Federal income tax benefit at statutory rate	$	(24,876	)	$	(21,556	)	$	(7,756	)
State income tax benefit, net of federal benefit		(7,756	)		(1,617	)		(1,511	)
Step-up in assets upon conversion to C-corporation		—			—			(709	)
Research and development credits		(6,297	)		(7,275	)		—
Stock-based compensation expense for incentive stock options and employee stock purchase plan		(3,012	)		1,805			283
Other non-deductible expenses and reconciling items		(26	)		170			129
Change in corporate tax rates		16,598			42			111
Change in valuation allowance		25,369			27,996			9,453
Total income tax expense (benefit)	$	—		$	(435	)	$	—

Components of Net Deferred Tax Assets

The significant components of the Company’s net deferred tax assets are as follows (in thousands):

	December 31, 2017			December 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$	37,658		$	25,625
Research and development tax credits		21,137			10,890
Step-up in assets upon conversion to C-corporation		721			953
Stock-based compensation expense for non-qualified stock options and restricted stock units		4,263			1,401
Unrealized losses on marketable securities		89			—
Deferred rent		458			258
Accruals and other		1,540			1,132
Total deferred tax assets before valuation allowance		65,866			40,259
Valuation allowance		(65,669	)		(40,084	)
Total deferred tax assets		197			175
Deferred tax liabilities:
Depreciation		(197	)		(19	)
Unrealized gains on marketable securities		—			(156	)
Total deferred tax liabilities		(197	)		(175	)
Net deferred tax assets	$	—		$	—

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Schedules for Computation of Diluted Weighted-Average Shares Outstanding

	Years Ended December 31,
	2017		2016		2015
Stock options issued and outstanding		5,468		5,118		3,684
Unvested restricted stock units outstanding		40		40		—
Employee stock purchase plan		20		22		—
		5,528		5,180		3,684

Supplemental Disclosures (Tables)

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Schedules of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consists of the following (in thousands):

	December 31, 2017		December 31, 2016
Accrued personnel costs	$	5,789	$	3,338
Accrued external general and administrative expenses		1,078		672
Accrued external research and development expenses		2,072		3,005
Accrued purchases of property and equipment		430		844
Other accrued expenses and current liabilities		236		267
	$	9,605	$	8,126

Selected Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information

	Quarters Ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Total revenues	$	455		$	6,562		$	1,336		$	2,040
Research and development expense	$	16,619		$	13,917		$	12,518		$	14,170
General and administrative expense	$	6,622		$	6,355		$	9,444		$	4,808
Total operating expenses	$	23,377		$	21,618		$	22,645		$	18,638
Net loss	$	(21,993	)	$	(14,473	)	$	(20,706	)	$	(15,997	)
Basic and diluted net loss per common share	$	(0.82	)	$	(0.47	)	$	(0.67	)	$	(0.51	)

	Quarters Ended
	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
Total revenues	$	393		$	2,375		$	125		$	1,696
Research and development expense	$	6,183		$	10,680		$	12,560		$	16,059
General and administrative expense	$	5,479		$	6,169		$	6,200		$	5,742
Total operating expenses	$	11,644		$	17,327		$	18,793		$	22,165
Net loss	$	(10,768	)	$	(14,437	)	$	(18,154	)	$	(19,608	)
Basic and diluted net loss per common share	$	(0.41	)	$	(0.55	)	$	(0.69	)	$	(0.74	)

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Mar. 27, 2017	Sep. 22, 2015	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Class of Stock [Line Items]
Net proceeds from issuance of common stock			$ 81,994
Accumulated deficit			(187,756)		$ (114,587)
Cash, cash equivalents and marketable securities			$ 176,400
Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued to during period			4,255,000	7,245,000
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		16,298,045		16,298,000
Initial Public Offering [Member]
Class of Stock [Line Items]
Number of shares issued to during period		7,245,000
Stock issuance price per share		$ 22.00
Initial Public Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock		$ 145,200
Overallotment Option [Member]
Class of Stock [Line Items]
Number of shares issued to during period		945,000
Overallotment Option [Member] \| Common Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Number of shares issued to during period	555,000
Follow-on Public Offering [Member]
Class of Stock [Line Items]
Number of shares issued to during period	3,700,000
Stock issuance price per share	$ 20.50
Follow-on Public Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 81,500

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Allowance for doubtful accounts receivable	$ 0	$ 0
Impairment losses	$ 0	0	$ 0
Percentage of maximum ownership interest under cost method investment	20.00%
Other-than-temporary impairment losses	$ 0	$ 0	$ 0
Estimated forfeiture rate on share-based awards granted		0.00%
Accrued interest or penalties related to uncertain tax positions	0	$ 0
Unrecognized tax benefits	0	0
Other Assets [Member]
Significant Accounting Policies [Line Items]
Cost method investments	$ 400,000	$ 0
Customer Concentration Risk [Member] \| United States [Member] \| Revenue [Member]
Significant Accounting Policies [Line Items]
Concentration risk percentage	96.00%	90.00%	78.00%
Customer Concentration Risk [Member] \| Europe [Member] \| Revenue [Member]
Significant Accounting Policies [Line Items]
Concentration risk percentage			21.00%

Summary of Significant Accounting Policies - Schedule of Revenue or Accounts Receivable by Major Customers (Detail) - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue [Member] \| Customer A [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	68.00%
Revenue [Member] \| Customer C [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage			41.00%
Revenue [Member] \| Customer D [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage		24.00%
Revenue [Member] \| Customer F [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage			16.00%
Revenue [Member] \| Customer G [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage			22.00%
Revenue [Member] \| Customer H [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage		44.00%
Accounts Receivable \| Customer B [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage		57.00%
Accounts Receivable \| Customer E [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	42.00%	19.00%
Accounts Receivable \| Customer G [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	32.00%
Accounts Receivable \| Customer I [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	21.00%

Summary of Significant Accounting Policies - Summary of Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	3 years
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	5 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	Shorter of lease term or estimated useful life

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 130,018	$ 133,724
Unrealized Gains	2	700
Unrealized Losses	(282)	(298)
Fair Value	129,738	134,126
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		300
Unrealized Gains		618
Fair Value		918
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	130,018	133,424
Unrealized Gains	2	82
Unrealized Losses	(282)	(298)
Fair Value	$ 129,738	$ 133,208

Marketable Securities - Additional Information (Detail)	12 Months Ended
Marketable Securities - Additional Information (Detail)	Dec. 31, 2017 USD ($) Bond	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities, net of tax	$ (682,000)	$ 686,000	$ (719,000)
Available for sale securities remaining maturities greater than three years	0	0
Unrealized gains (losses) on available-for-sale securities, before tax	(200,000)	1,100,000	(700,000)
Unrealized gains (losses) on available-for-sale securities, income tax expense	0	400,000	0
Net realized gains or losses available for sale securities	479,000
Aggregate fair value of securities in an unrealized loss position for less than twelve months	109,200,000	102,500,000
Aggregate fair value of securities in an unrealized loss position for more than twelve months	17,100,000
Aggregate unrealized loss of securities in an unrealized loss position for more than twelve months		0
Other-than-temporary impaired	$ 0	0
Number of investment grade corporate bonds \| Bond	46
Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Aggregate unrealized loss of securities in an unrealized loss position for more than twelve months	$ 100,000
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities, net of tax	600,000
Common Equity Securities [Member] \| Audentes Therapeutics, Inc. [Member] \| Investment Income
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 500,000
Common Equity Securities [Member] \| Audentes Therapeutics, Inc. [Member] \| Investment Income \| Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities		$ (100,000)	$ 0

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 129,738	$ 134,126
Assets fair value disclosure	176,384	158,966
Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities		918
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	129,738	133,208
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Cash equivalents	46,646	24,840
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure		918
Quoted Prices in Active Markets (Level 1) [Member] \| Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities		918
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	176,384	158,048
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	129,738	133,208
Significant Other Observable Inputs (Level 2) [Member] \| Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Cash equivalents	$ 46,646	$ 24,840

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair value hierarchy level 1 to level 2 transfers amount	$ 0	$ 0
Fair value hierarchy level 2 to level 1 transfers amount	$ 0	$ 0

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 17,254	$ 9,948
Accumulated depreciation and amortization	(3,277)	(624)
Property and equipment, net	13,977	9,324
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,481	1,038
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	8,561	2,780
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,384	1,213
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 5,828	$ 4,917

Property and Equipment Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation or amortization expense	$ 2,686	$ 544	$ 80

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended							9 Months Ended	12 Months Ended			38 Months Ended	106 Months Ended
	Nov. 30, 2016	May 31, 2016	Jan. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Nov. 30, 2014	Mar. 31, 2009	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2017
Other Commitments [Line Items]
Tenant improvement allowance received				$ 400,000
Restricted cash as collateral with financial institution									$ 225,000	$ 225,000		$ 225,000	$ 225,000
Operating Leases, Rent Expense, Net									1,800,000	1,800,000	$ 400,000
Total expenses under license agreement									1,703,000	861,000	1,405,000
Claims paid to date related to indemnification issues									0
Accruals or expenses related to indemnification issues									0	0
European Patent Office Proceeding [Member]
Other Commitments [Line Items]
Liability related to third party opposition									$ 0			0	0
The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement									24.50%
Accrued expenses										200,000
The Trustees of the University of Pennsylvania [Member] \| Maximum [Member]
Other Commitments [Line Items]
Accrued expenses									$ 100,000			100,000	100,000
The Trustees of the University of Pennsylvania [Member] \| License Fees [Member]
Other Commitments [Line Items]
Related party transaction expense									444,000	574,000	866,000
The Trustees of the University of Pennsylvania [Member] \| License Fees [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Related party transaction expense								$ 400,000	207,000	430,000	703,000
The Trustees of the University of Pennsylvania [Member] \| License Fees [Member] \| Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Related party transaction expense								400,000	4,000	11,000	9,000
The Trustees of the University of Pennsylvania [Member] \| License Fees [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Related party transaction expense								400,000	233,000	133,000	154,000
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement							19.90%
Accrued expenses									300,000	400,000		300,000	300,000
Related party transaction expense									2,008,000	1,034,000	1,281,000
Milestone payment obligation							$ 1,500,000
GlaxoSmithKline LLC [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Related party transaction expense								700,000	997,000	430,000	703,000
GlaxoSmithKline LLC [Member] \| Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Related party transaction expense								700,000	2,000	7,000	5,000
GlaxoSmithKline LLC [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Related party transaction expense								$ 700,000	509,000	597,000	573,000
GlaxoSmithKline LLC [Member] \| Milestone Fees [Member]
Other Commitments [Line Items]
Related party transaction expense									500,000				500,000
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Total expenses under license agreement									100,000	100,000
Amounts due under agreement									$ 100,000			100,000	$ 100,000
Regents of the University of Minnesota [Member] \| Maximum [Member]
Other Commitments [Line Items]
Up-front fee paid and patent maintenance expenses reimbursed						$ 100,000
Up-front fee paid	$ 100,000
Total expenses under license agreement											$ 100,000
Amounts due under agreement										$ 100,000
Regents of the University of Minnesota [Member] \| Milestone Fees [Member] \| Maximum [Member]
Other Commitments [Line Items]
Related party transaction expense												$ 100,000
Regents of the University of Minnesota [Member] \| Annual License Maintenance Fee [Member] \| Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						100,000
Regents of the University of Minnesota [Member] \| Royalties And Sublicense Fees [Member] \| Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						$ 100,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease commenced date			2016-02		2015-04
Lease term									September 2015, November 2015 and July 2017: Ultimately extend the term of the lease to September 2021.
Options to extend the additional lease term				6 years
Tenant improvement allowance received			$ 800,000
Lease period			7 years 6 months
Lease expiration date			Sep. 30, 2023
Options to extend the additional lease term			5 years
Monthly lease payments commencement period			2016-09
New York, New York [Member]
Other Commitments [Line Items]
Lease commenced date		2016-07
Lease expiration date		Oct. 31, 2020
Monthly lease payments commencement period		2016-10
Lease expiration term		51 months
Letters of credit		$ 200,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2018	$ 2,172
2019	2,297
2020	2,310
2021	1,720
2022	621
Thereafter	479
Total minimum lease payments	$ 9,599

Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended			106 Months Ended
	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense		$ 2,008	$ 1,034	$ 1,281
GlaxoSmithKline LLC [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 700	997	430	703
GlaxoSmithKline LLC [Member] \| Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense	700	2	7	5
GlaxoSmithKline LLC [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	700	509	597	573
GlaxoSmithKline LLC [Member] \| Milestone Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense		500			$ 500
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense		444	574	866
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	400	207	430	703
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense	400	4	11	9
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 400	$ 233	$ 133	$ 154

Capitalization - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	May 15, 2015	Jan. 13, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Capitalization [Line Items]
Common stock, shares authorized			100,000,000	100,000,000
Common stock, par value			$ 0.0001	$ 0.0001
Preferred stock, shares authorized			10,000,000	10,000,000
Preferred stock, par value			$ 0.0001	$ 0.0001
Convertible preferred stock, issuance cost			$ 445	$ 60
Net gain on extinguishment of convertible preferred stock					$ 800
Closing date of IPO			Sep. 22, 2015
Outstanding convertible preferred stock into common stock			16,298,045
Series C Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, shares sale and issuance		4,631,774
Convertible preferred stock, par value		$ 0.000001
Convertible preferred stock, per share price		$ 6.477
Gross proceeds from sale and issuance of convertible preferred stock		$ 30,000			26,021
Proceeds from exercise of stock options		26,000
Convertible preferred stock, issuance cost		200
Convertible preferred stock, conversion of other debt		$ 3,800
Series A Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Net gain on extinguishment of convertible preferred stock					(1,300)
Series B Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Net gain on extinguishment of convertible preferred stock					2,100
Series D Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, shares sale and issuance	7,366,849
Convertible preferred stock, par value	$ 0.0001
Convertible preferred stock, per share price	$ 9.5699
Gross proceeds from sale and issuance of convertible preferred stock	$ 68,000				$ 67,998
Convertible preferred stock, issuance cost	$ 2,500
Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, shares outstanding			0	0

Capitalization - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail) - shares shares in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Capitalization [Line Items]
Reserved common shares	7,636	7,141
Equity Incentive Plans [Member]
Schedule of Capitalization [Line Items]
Reserved common shares	7,430	6,887
Employee Stock Purchase Plan [Member]
Schedule of Capitalization [Line Items]
Reserved common shares	206	254

Significant Agreements - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Agreements [Line Items]
Milestone fee upon commencement of clinical trials in humans			$ 27,800
Milestone fee upon submission of regulatory approval filings			48,500
Milestone fee upon approval of commercial products by regulatory agencies			110,500
Milestone fee upon achievement of specified sales targets for licensed products			92,000
License revenue			10,385	$ 4,303	$ 5,025
January 2018 Amendment Agreement [Member]
Significant Agreements [Line Items]
License revenue			0
January 2018 Amendment Agreement [Member] \| Subsequent Event [Member]
Significant Agreements [Line Items]
License fee	$ 80,000
Milestone payment unpaid in the event of change of control	40,000
January 2018 Amendment Agreement [Member] \| Maximum [Member] \| Subsequent Event [Member]
Significant Agreements [Line Items]
Milestone payment	120,000
January 2018 Amendment Agreement [Member] \| First Anniversary [Member] \| Subsequent Event [Member]
Significant Agreements [Line Items]
License fee	30,000
January 2018 Amendment Agreement [Member] \| Second Anniversary [Member] \| Subsequent Event [Member]
Significant Agreements [Line Items]
License fee	$ 30,000
March 2014 License Agreement [Member]
Significant Agreements [Line Items]
License revenue			$ 1,100	$ 100	$ 300
AveXis, Inc. [Member]
Significant Agreements [Line Items]
License agreement amendment date			2018-01
License revenue			$ 6,000
License up-front fee		$ 6,000
Milestone fee payments upon achievement of various development and commercialization		$ 36,000

Significant Agreements - Schedule of License Revenue (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
License Agreement Revenue Recognition [Abstract]
Up-front fees and option fees for commercial licenses	$ 6,420	$ 3,050	$ 5,900
Maintenance fees for commercial licenses	920	910	635
Milestone fees	1,850		250
Research and other license revenue	1,195	343	240
Licenses revenue including amounts from related parties	$ 10,385	$ 4,303	$ 7,025

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			12 Months Ended
	Sep. 16, 2015	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense					$ 22,900
Unrecognized stock-based compensation, weighted-average period					2 years 6 months
Proceeds from exercise of stock options					$ 2,493	$ 179	$ 107
Stock-based compensation expense					$ 10,605	$ 7,031	$ 2,921
Additional shares to be authorized					7,636,000	7,141,000
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of annual increase in number of shares available for future issuance					4.00%
Additional shares to be issued			1,059,065	1,052,538
Common stock shares authorized for issuance					8,236,603
Shares available for future grants					1,922,892
Stock options expiration date					10 years
Stock options vesting period					4 years
Stock plan expiration date					2025-06
Weighted-average fair values of options granted					$ 13.87	$ 8.37	$ 7.08
Exercise of stock options, Shares					515,916	163,158	125,563
Total intrinsic value of options exercised					$ 11,100	$ 1,700	$ 500
2015 Equity Incentive Plan [Member] \| Employee and Director Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense					10,100	6,400	1,400
2015 Equity Incentive Plan [Member] \| Non Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense					$ 500	$ 600	$ 1,500
2015 Equity Incentive Plan [Member] \| Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued		1,251,810
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance	254,000				254,000
Shares available for future grants					206,280
Share-based compensation arrangement by share-based payment award, description					The number of shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year, commencing in 2016
Share-based compensation arrangement by share-based payment award, percentage of outstanding stock minimum	1.00%
Additional shares to be authorized			0	0
Common stock shares issued to participants					47,720	0	0

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 10,605	$ 7,031	$ 2,921
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	10,031	6,950	$ 2,921
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	275	23
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 299	$ 58

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 10,605	$ 7,031	$ 2,921
Research and Development Costs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	5,128	2,743	1,760
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,477	$ 4,288	$ 1,161

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,118
Shares, Granted	1,155
Shares, Exercised	(516)
Shares, Cancelled or forfeited	(289)
Shares Outstanding, Ending Balance	5,468	5,118
Shares, Exercisable	2,979
Shares, Vested and expected to vest	5,468
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 7.77
Weighted-average Exercise Price, Granted	20.44
Weighted-average Exercise Price, Exercised	4.83
Weighted-average Exercise Price, Cancelled or forfeited	16.59
Weighted-average Exercise Price, Outstanding, Ending Balance	10.25	$ 7.77
Weighted-average Exercise Price, Exercisable	6.24
Weighted-average Exercise Price, Vested and expected to vest	$ 10.25
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 10 months 24 days	8 years 6 months
Weighted-average Remaining Contractual Life (Years), Exercisable	7 years 4 months 24 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 10 months 24 days
Aggregate Intrinsic Value Outstanding	$ 125,738	$ 57,400
Aggregate Intrinsic Value, Exercisable	80,482
Aggregate Intrinsic Value, Vested and expected to vest	$ 125,738

Stock-based Compensation - Schedule of Employees Stock Purchase Plan Valuation Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee and Director Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	77.00%	75.00%	68.00%
Expected term (in years)	6 years 2 months 12 days	6 years 2 months 12 days	6 years 1 month 6 days
Risk-free interest rate	2.10%	1.50%	1.70%
Expected dividend yield	0.00%	0.00%	0.00%
Non Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	77.00%	83.00%	75.00%
Expected term (in years)	7 years 8 months 12 days	8 years 6 months	9 years 6 months
Risk-free interest rate	2.20%	1.70%	2.10%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Unvested, Beginning Balance	40
Shares, Granted	0
Shares, Vested	0
Shares, Forfeited	0
Shares Unvested, Ending Balance	40
Weighted-average Grant Date Fair Value, Unvested Beginning Balance \| $ / shares	$ 20.90
Weighted-average Grant Date Fair Value, Unvested Ending Balance \| $ / shares	$ 20.90

Retirement Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Contribution by company	$ 1.0	$ 0.6	$ 0.2
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Company's matching contribution, percentage of employee deferrals	5.75%

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Income tax expense (benefit), foreign		$ 0	$ 0	$ 0
Federal statutory income tax rate, percent		34.00%
Tax Cuts and Jobs Act of 2017, limitation of net operating losses deductions as percentage of current year taxable income		80.00%
Increase in valuation allowance		$ 25,600,000	28,000,000
Income tax effects of gains on available-for-sale securities recognized in other comprehensive income		0	400,000	$ 0
Research and development and orphan drug tax credit carryforwards		21,137,000	10,890,000
Reserve for uncertain tax positions placed against credit carryforwards		100,000
Reserve for uncertain tax positions placed against qualified expenses		$ 0	0
Cumulative changes in ownership Interest percentage threshold, subject to annual limitation of NOL and tax credit carryforwards		50.00%
Ownership interest change period		3 years
Beginning Tax Year [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards, expiration year		2035
Domestic Country [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards		$ 126,700,000	66,400,000
Research and development and orphan drug tax credit carryforwards		$ 21,300,000
Domestic Country [Member] \| Beginning Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating Loss Carryforwards Expiration Year		2034
U. S State Tax Authority [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards		$ 167,800,000	$ 55,000,000
U. S State Tax Authority [Member] \| Beginning Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating Loss Carryforwards Expiration Year		2035
Scenario Forecast [Member]
Income Tax Disclosure [Line Items]
Federal statutory income tax rate, percent	21.00%

Income Taxes - Schedule of Reconciliation of Effective Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax benefit at statutory rate	$ (24,876)	$ (21,556)	$ (7,756)
State income tax benefit, net of federal benefit	(7,756)	(1,617)	(1,511)
Step-up in assets upon conversion to C-corporation			(709)
Research and development credits	(6,297)	(7,275)
Stock-based compensation expense for incentive stock options and employee stock purchase plan	(3,012)	1,805	283
Other non-deductible expenses and reconciling items	(26)	170	129
Change in corporate tax rates	16,598	42	111
Change in valuation allowance	25,369	27,996	9,453
Total income tax expense (benefit)	$ 0	$ (435)	$ 0

Income Taxes - Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 37,658	$ 25,625
Research and development tax credits	21,137	10,890
Step-up in assets upon conversion to C-corporation	721	953
Stock-based compensation expense for non-qualified stock options and restricted stock units	4,263	1,401
Unrealized losses on marketable securities	89
Deferred rent	458	258
Accruals and other	1,540	1,132
Total deferred tax assets before valuation allowance	65,866	40,259
Valuation allowance	(65,669)	(40,084)
Total deferred tax assets	197	175
Deferred tax liabilities:
Depreciation	(197)	(19)
Unrealized gains on marketable securities		(156)
Total deferred tax liabilities	(197)	(175)
Net deferred tax assets	$ 0	$ 0

Related Party Transactions - Additional Information (Detail) - USD ($)		4 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2015	Apr. 30, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
License revenue from license agreements				$ 10,385,000	$ 4,303,000	$ 5,025,000
Preferred stock, par value				$ 0.0001	$ 0.0001
Rent expense				$ 1,800,000	$ 1,800,000	400,000
The Trustees of the University of Pennsylvania [Member] \| Manufacturing of Reagents for Sale [Member]
Related Party Transaction [Line Items]
Related party transaction expense			$ 100,000
The Trustees of the University of Pennsylvania [Member] \| Research and Development Services [Member]
Related Party Transaction [Line Items]
Related party transaction expense			5,100,000
Dimension Therapeutics Inc
Related Party Transaction [Line Items]
License revenue from license agreements			2,000,000
Amount received for purchase of materials from related party							$ 200,000
Gain on disposal of the material to related party							$ 200,000
Gains related to delivery of purchased material			$ 100,000
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Interest expense	$ 100
Monthly lease payments	$ 100,000
Rent expense		$ 100,000
Lease expiration date	Apr. 30, 2015
FoxKiser,LLP [Member] \| Series C Preferred Stock
Related Party Transaction [Line Items]
Issuance of convertible preferred stock for the conversion of outstanding related party debt	$ 3,800,000
Issuance of convertible preferred stock for the conversion of outstanding related party debt, Shares	585,578
Preferred stock, par value	$ 6.477
FoxKiser,LLP [Member] \| Service Agreements [Member]
Related Party Transaction [Line Items]
Related party transaction, description				The amounts outstanding to FOXKISER under the services agreement in excess of 30 days from their due date accrued interest at 1.5% per month, compounding monthly.
Accrued interest compounding monthly on amount outstanding				1.50%
FoxKiser,LLP [Member] \| Service Agreements [Member] \| Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense				$ 1,500,000	1,000,000	200,000
FoxKiser,LLP [Member] \| Service Agreements [Member] \| General and Administrative [Member]
Related Party Transaction [Line Items]
Related party transaction expense						200,000
Chief Scientific Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees				$ 100,000	$ 300,000	$ 200,000
Shares, Granted						211,600,000	211,600,000

Net Loss Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	5,528	5,180	3,684
Unvested Restricted Stock Units Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	40	40
Stock Options Issued and Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	5,468	5,118	3,684
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	20	22

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued personnel costs	$ 5,789	$ 3,338
Accrued external general and administrative expenses	1,078	672
Accrued external research and development expenses	2,072	3,005
Accrued purchases of property and equipment	430	844
Other accrued expenses and current liabilities	236	267
Accrued expenses and other current liabilities	$ 9,605	$ 8,126

Selected Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,040	$ 1,336	$ 6,562	$ 455	$ 1,696	$ 125	$ 2,375	$ 393	$ 10,393	$ 4,589	$ 7,588
Research and development expense	14,170	12,518	13,917	16,619	16,059	12,560	10,680	6,183	57,224	45,482	17,279
General and administrative expense	4,808	9,444	6,355	6,622	5,742	6,200	6,169	5,479	27,229	23,590	11,912
Total operating expenses	18,638	22,645	21,618	23,377	22,165	18,793	17,327	11,644	86,278	69,929	30,725
Net loss	$ (15,997)	$ (20,706)	$ (14,473)	$ (21,993)	$ (19,608)	$ (18,154)	$ (14,437)	$ (10,768)	$ (73,169)	$ (62,967)	$ (22,811)
Basic and diluted net loss per common share	$ (0.51)	$ (0.67)	$ (0.47)	$ (0.82)	$ (0.74)	$ (0.69)	$ (0.55)	$ (0.41)	$ (2.45)	$ (2.38)	$ (2.59)