Spark Therapeutics (ONCE) 10-K 14 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Mar. 04, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	1,609,351
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-K
Trading Symbol	ONCE
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus (Q1,Q2,Q3,FY)	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		27,109,724
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 779

Balance sheets

Balance sheets - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 293,530,590	$ 74,566,963
Other receivables	16,944,568	244,393
Prepaid expenses and other current assets	1,132,626	2,551,912
Total current assets	311,607,784	77,363,268
Property and equipment, net	16,999,445	12,674,372
Other assets	1,165,285	408,211
Total assets	329,772,514	90,445,851
Current liabilities:
Accounts payable	9,687,594	2,676,697
Accrued expenses	6,529,263	3,163,154
Current portion of deferred rent	715,959	0
Current portion of deferred revenue	5,182,835	10,014,377
Total current liabilities	22,115,651	15,854,228
Long-term deferred rent	8,084,509	8,618,489
Long-term deferred revenue	9,034,559	10,767,414
Total liabilities	39,234,719	35,240,131
Stockholders’ equity:
Preferred stock, value	0	0
Common stock, $0.001 par value. Authorized, 150,000,000 shares; issued and outstanding, 6,290,317 shares at December 31, 2014; 27,082,493 shares issued and 27,073,287 outstanding at December 31, 2015	27,083	6,290
Additional paid-in capital	419,791,732	54,364,833
Treasury stock, at cost 9,206 shares at December 31, 2015	(552,636)	0
Accumulated deficit	(128,728,384)	(81,602,606)
Total stockholders’ equity	290,537,795	55,205,720
Total liabilities and stockholders’ equity	329,772,514	90,445,851
Series A Convertible Preferred Stock [Member]
Stockholders’ equity:
Preferred stock, value	0	10,000,000
Series B Convertible Preferred Stock [Member]
Stockholders’ equity:
Preferred stock, value	$ 0	$ 72,437,203

Balance sheets (unaudited) (Par

Balance sheets (unaudited) (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
Preferred stock , par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	27,082,493	6,290,317
Common stock, shares outstanding	27,073,287	6,290,317
Treasury stock, shares	9,206	0
Series A Convertible Preferred Stock [Member]
Preferred stock , par value		$ 0.001
Preferred stock, shares authorized		5,000,000
Preferred stock, shares issued		5,000,000
Preferred stock, shares outstanding		5,000,000
Series B Convertible Preferred Stock [Member]
Preferred stock , par value		$ 0.001
Preferred stock, shares authorized		45,186,334
Preferred stock, shares issued		45,186,334
Preferred stock, shares outstanding		45,186,334

Statements of operations

Statements of operations - USD ($)	10 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenues	$ 0	$ 22,063,674	$ 633,932
Operating expenses:
Research and development	4,897,152	46,029,314	16,351,005
Acquired in-process research and development	50,000,000	0	750,000
General and administrative	2,380,645	23,352,171	7,863,256
Total operating expenses	57,277,797	69,381,485	24,964,261
Loss from operations	(57,277,797)	(47,317,811)	(24,330,329)
Interest income	0	192,033	5,520
Net loss	(57,277,797)	(47,125,778)	(24,324,809)
Preferred stock dividends	0	(634,794)	(707,342)
Net loss applicable to common stockholders	$ (57,277,797)	$ (47,760,572)	$ (25,032,151)
Basic and diluted net loss per common share (in usd per share)	$ (8.44)	$ (2.10)	$ (4.64)
Weighted average basic and diluted common shares outstanding (in shares)	6,788,396	22,710,105	5,397,599

Statement of stockholders' equi

Statement of stockholders' equity - USD ($)	Total	Member Units [Member]	Member Units [Member]Series A Convertible Preferred Stock [Member]	Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Preferred Stock [Member]Series A Convertible Preferred Stock [Member]Initial Public Offering [Member]	Preferred Stock [Member]Series B Convertible Preferred Stock [Member]	Preferred Stock [Member]Series B Convertible Preferred Stock [Member]Initial Public Offering [Member]	Treasury Stock [Member]	Common Stock [Member]	Common Stock [Member]Series A Convertible Preferred Stock [Member]Initial Public Offering [Member]	Common Stock [Member]Series B Convertible Preferred Stock [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Series A Convertible Preferred Stock [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]Series B Convertible Preferred Stock [Member]Initial Public Offering [Member]	Accumulated Deficit [Member]
Beginning balance, common units at Dec. 31, 2013	$ 3,368,788	$ 50,646,585	$ 10,000,000									$ 0			$ (57,277,797)
Beginning balance, preferred units (in shares) at Dec. 31, 2013			5,000,000
Beginning balance, common units (in shares) at Dec. 31, 2013		30,870,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common units in connection with license and technology agreements	750,000								$ 200			749,800
Issuance of common units in connection with license and technology agreements (in shares)									200,000
Issuance of convertible preferred stock/units						$ 72,437,203
Issuance of convertible preferred stock/units (in shares/units)						45,186,334
Conversion from LLC to C corporation	0	$ (50,646,585)	$ (10,000,000)	$ 10,000,000					$ 6,090			50,640,495
Conversion from LLC to C corporation (in shares)		(31,910,667)	(5,000,000)	5,000,000					6,090,317
Issuance of restricted stock/units, net of forfeitures(in shares)		1,040,667
Stock-based compensation expense	2,974,538											2,974,538
Net loss	(24,324,809)														(24,324,809)
Ending balance, common stock at Dec. 31, 2014	55,205,720			$ 10,000,000		$ 72,437,203			$ 6,290			54,364,833			(81,602,606)
Ending balance, preferred units (in shares) at Dec. 31, 2014			0
Ending balance, common units (in shares) at Dec. 31, 2014		0
Ending balance, common stock (in shares) at Dec. 31, 2014				5,000,000		45,186,334			6,290,317
Beginning balance, common units at Dec. 31, 2014	55,205,720	$ 0	$ 0	$ 10,000,000		$ 72,437,203			$ 6,290			54,364,833			(81,602,606)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of convertible preferred stock/units	268,321,943								$ 10,317			268,311,626
Issuance of convertible preferred stock/units (in shares/units)									10,316,995
Conversion of preferred stock and dividends to common stock upon initial public offering					$ (10,000,000)		$ (72,437,203)			$ 1,016	$ 9,184		$ 9,998,984	$ 72,428,019
Conversion of preferred stock and dividends to common stock upon initial public offering (in shares)					(5,000,000)		(45,186,334)			1,016,219	9,183,831
Issuance of common stock for services	193,909								$ 4			193,905
Issuance of common stock for services (in shares)									3,556
Issuance of restricted stock									$ 50			(50)
Issuance of restricted stock/units, net of forfeitures(in shares)									49,750
Purchase of common stock in treasury	(552,636)							$ (552,636)
Purchase of common stock in treasury (in shares)								9,206
Exercise of stock options	$ 1,116,043								$ 222			1,115,821
Exercise of stock options (in shares)	221,825								221,825
Stock-based compensation expense	$ 13,378,594											13,378,594
Net loss	(47,125,778)														(47,125,778)
Ending balance, common stock at Dec. 31, 2015	$ 290,537,795			$ 0		$ 0		$ (552,636)	$ 27,083			$ 419,791,732			$ (128,728,384)
Ending balance, common stock (in shares) at Dec. 31, 2015				0		0		(9,206)	27,082,493

Statements of cash flows

Statements of cash flows - USD ($)	10 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (57,277,797)	$ (47,125,778)	$ (24,324,809)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Noncash rent expense	0	181,979	617,114
Depreciation and amortization expense	0	1,732,983	169,790
Acquired in-process research and development	50,000,000	0	750,000
Stock-based compensation expense	646,585	13,572,503	2,974,538
Changes in operating assets and liabilities:
Prepaid expenses and other assets	0	(1,627,602)	(670,309)
Accounts payable and accrued expenses	1,492,497	9,052,260	2,331,282
Deferred rent	0	0	7,901,375
Deferred revenue	0	(6,564,397)	20,781,791
Net cash (used in) provided by operating activities	(5,138,715)	(47,478,227)	10,386,379
Cash flows from investing activities:
Purchases of property and equipment	0	(4,521,769)	(11,696,962)
Net cash used in investing activities	0	(4,521,769)	(11,696,962)
Cash flows from financing activities:
Proceeds from issuance of Series A convertible preferred units	5,138,715	0	4,861,285
Proceeds from issuance of Series B convertible preferred stock, net	0	0	72,437,203
Financing costs	0	0	(1,420,942)
Repurchase of common stock	0	(552,636)	0
Proceeds from exercise of options	0	1,116,043	0
Proceeds from public offerings of common stock, net	0	270,400,216	0
Net cash provided by financing activities	5,138,715	270,963,623	75,877,546
Net increase in cash and cash equivalents	0	218,963,627	74,566,963
Cash and cash equivalents, beginning of period		74,566,963	0
Cash and cash equivalents, end of period	0	293,530,590	74,566,963
Supplemental disclosure of cash flow information:
Deferred financing costs included in accounts payable and accrued expenses	0	657,331	868,872
Property and equipment purchases included in accounts payable and accrued expenses	$ 0	$ 2,683,487	$ 1,147,200

Background

Background	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Background	Background Spark Therapeutics, Inc. was formed on March 13, 2013 in the state of Delaware as AAVenue Therapeutics, LLC and amended its Certificate of Formation in October 2013 to change its name to Spark Therapeutics LLC. In May 2014, the Company converted from a limited liability company (LLC) to a C corporation, Spark Therapeutics, Inc. (the Company). The Company is a gene therapy company, seeking to transform the lives of patients suffering from debilitating genetic diseases by developing one-time, life-altering treatments. The Company operates in one segment and has its principal offices in Philadelphia, Pennsylvania. (a) Initial Public Offering (IPO) On February 4, 2015 , the Company closed its IPO, having sold 8,050,000 shares of common stock at an IPO price of $23.00 per share, for aggregate gross proceeds of $185.2 million . The Company received net proceeds from the IPO of $168.9 million , after deducting underwriting discounts and commissions and other offering expenses. As part of the IPO, all of the outstanding shares of preferred stock, including shares of preferred stock issued as accrued dividends, were converted into an aggregate of 10,200,050 shares of common stock. On December 28, 2015, the Company completed its follow-on public offering, having sold 2,266,995 shares of common stock at a offering price of $47.00 per shares, for aggregate gross proceeds of $106.5 million . The Company received net proceeds from the public offering of $99.4 million , after deducting underwriting discounts and commissions and other offering expenses.

Development-stage risks

Development-stage risks	12 Months Ended
	Dec. 31, 2015
Development-stage Risk [Abstract]
Development-stage risks	Development-stage risks The Company has incurred losses and negative cash flows from operations since inception and had an accumulated deficit of $128.7 million at December 31, 2015 . The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates in development. Additional financing may be needed by the Company to fund its operations and to commercially develop its product candidates. The Company’s future operations are highly dependent on a combination of factors, including: (i) the success of its research and development; (ii) regulatory approval and market acceptance of the Company’s proposed future products; (iii) the timely and successful completion of additional financing; and (iv) the development of competitive therapies by other biotechnology and pharmaceutical companies.

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Use of estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates. (b) Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, other receivables and accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments. (c) Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of December 31, 2014 and 2015 consisted primarily of money market funds. (d) Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded 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, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment 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 undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. No impairment charges have been recorded since inception. (e) Research and development and in-process research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use. (f) Income taxes From inception through May 1, 2014, the Company was a Delaware LLC for federal and state tax purposes and, therefore, all items of income or loss through May 1, 2014 flowed through to the members of the LLC. Effective May 2, 2014, the Company converted from an LLC to a C corporation for federal and state income tax purposes. Accordingly, prior to the conversion to a C corporation, the Company did not record deferred tax assets or liabilities or have any net operating loss carryforwards. The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes, if any, is applied during the years in which temporary differences are expected to be settled and is reflected in the financial statements in the period of enactment. The measurement of deferred tax assets is reduced, if necessary, if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2015, the Company has concluded that a full valuation allowance is necessary for its deferred tax assets. (g) Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Topic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: • the milestone is commensurate with either: (1) the performance required to achieve the milestone or (2) the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; • the milestone relates solely to past performance; and • the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. To date, the Company has not generated any revenues from the commercial sale of products. (h) Stock-based compensation and fair value of stock 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 and modifications to existing stock options, to be recognized in the statements of operations based on their fair values. The Company uses the Black-Scholes option pricing model to determine the fair value of options granted. The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and directors with service-based vesting conditions is recognized on a straight-line basis based on the 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, using the accelerated attribution method. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through share-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed over the vesting period. The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option pricing model, which requires the input of and subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate, and (iv) 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 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 will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. 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. The Company also is required to estimate forfeitures at the time of grant, and revise those estimates in subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate pre-vesting option forfeitures and records stock-based compensation expense only for those awards that are expected to vest. 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. Stock-based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest. 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 and restricted stock award 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 over the contractual life. (i) Recapitalization On January 16, 2015, the Company effected a reverse stock split of the Company’s common stock at a ratio of one share for every five shares previously held. All common stock share and common stock per share data included in these financial statements reflect the reverse stock split. (j) Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented, the outstanding shares of convertible preferred stock, unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities have been excluded from the computations of diluted weighted average units/shares outstanding as of December 31, 2014 and 2015 as they would be anti-dilutive: Period from March 13, 2013 (inception) to December 31, December 31, 2013 2014 2015 Convertible preferred units 5,000,000 — — Unvested restricted common units 5,370,000 — — Convertible preferred shares — 10,037,255 — Unvested restricted common shares — 578,994 373,655 Options issued and outstanding — 2,264,497 3,071,372 Amounts in the table above reflect the common stock equivalents of the noted instruments. (k) Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property, which was May 1, 2014 for the Company’s corporate headquarters. Tenant improvement allowances from the lessor are included in the accompanying balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of December 31, 2015 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received. (l) Recent Accounting Pronouncements In November 2015, the FASB issued ASU 2015-17, “Balance Sheet Classification of Deferred Taxes”, which requires that deferred tax assets and liabilities be classified as noncurrent in a classified statement of financial position. This update is effective for financial statements issued for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years, and may be applied either prospectively to all deferred tax assets and liabilities or retrospectively to all periods presented. The Company elected to early-adopt this update on a retrospective basis. The adoption of this update had no effect on the Company's results of operations. In February 2016, the FASB issued ASU 2016-02, “Leases.” ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. The updated guidance is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company has not evaluated the impact of the updated guidance on the Company's financial statements.

Fair value of financial instrum

Fair value of financial instruments	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair value of financial instruments	Fair value of financial instruments The Company follows FASB accounting guidance on fair value measurements for financial assets and liabilities measured on a recurring basis. The guidance requires fair value measurements to maximize the use of “observable inputs.” The three-level hierarchy of inputs to measure fair value are as follows: • Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities • Level 2: Significant other observable inputs other than Level 1 prices such as quoted prices in markets that are not active, or inputs that are observable, either directly or indirectly, for substantially the full term of the asset or liability • Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity) The Company has classified assets and liabilities measured at fair value on a recurring basis as follows: Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2014: Assets: Money market funds (included in cash and cash equivalents) $ 74,025,841 — — At December 31, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 293,530,590 — —

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Property and equipment, net	Property and equipment, net Property and equipment consist of the following: December 31, 2014 2015 Laboratory equipment $ 2,961,494 $ 4,415,121 Computer and software 52,710 158,280 Furniture and fixtures 290,818 470,151 Office equipment 59,635 72,308 Leasehold improvements 5,113,269 11,274,721 Construction in progress 4,366,236 2,511,636 Property and equipment, gross 12,844,162 18,902,217 Less accumulated depreciation and amortization (169,790 ) (1,902,772 ) Property and equipment, net $ 12,674,372 $ 16,999,445 Depreciation and amortization expense was $0.2 million and $1.7 million for the years ended December 31, 2014 and 2015, respectively.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2015
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following: December 31, 2014 2015 Compensation and benefits $ 1,385,013 $ 4,880,239 Consulting and professional fees 1,327,942 432,346 Research and development 247,448 978,156 Other 202,751 238,522 $ 3,163,154 $ 6,529,263

Stockholders' equity

Stockholders' equity	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Stockholders' equity	Stockholders’ equity The Company’s certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. The Company has authorized 150,000,000 shares of common stock and 5,000,000 shares of preferred stock. (a) Convertible preferred October 2013 Series A financing In October 2013, the Company entered into an agreement with The Children’s Hospital of Philadelphia (CHOP) to sell 5,000,000 Series A Units at $2.00 per unit for proceeds of $10.0 million . Each Series A Unit was convertible into one Series 1 Unit (subject to certain antidilution adjustments) at any time at the option of the holder. The Series A Units were mandatorily convertible into common stock in the event of an IPO, as defined. May 2014 conversion to C corporation Upon conversion of the Company into a C corporation in May 2014, each outstanding Series A Unit converted into one share of Series A convertible preferred stock (Series A Stock). May 2014 Series B financing In May 2014, the Company issued 45,186,334 shares of Series B convertible preferred stock (Series B Stock) for $72.4 million , net of transaction costs. In conjunction with the issuance of Series B Stock, certain Series A convertible preferred stock (Series A Stock) terms were amended. Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders, subject to certain customary antidilution adjustments contained in the Company’s certificate of incorporation. The Series A Stock and Series B Stock were entitled to receive cumulative dividends at 8% per annum, which accrued from day to day beginning November 23, 2014 and were payable upon conversion, an event of liquidation or a qualified IPO, in each case, in shares of Series A Stock and Series B Stock, as applicable. As of February 4, 2015, dividends of $1.3 million had accumulated, and in connection with the IPO, were declared and converted along with all outstanding shares of Series A Stock and Series B Stock into an aggregate of 10,200,050 shares of common stock. (b) Common Through May 1, 2014, the Board designated Series 1 Units, Series 2 Units and Series 3 common units (Series 3 Units). Upon conversion of the Company into a C corporation in May 2014, each outstanding Series 1 Unit converted into 0.2 shares (post-split) of common stock, each outstanding Series 2 Unit converted into 0.2 shares (post-split) of common stock and each outstanding Series 3 Unit converted into 0.03883773 shares (post-split) of common stock. In 2013 and 2014, the Company issued Series 2 Units and Series 3 Units to various employees, directors and consultants of the Company with specified vesting provisions. In connection with the conversion to a C corporation, the units converted to common stock. The vesting terms of the common stock vary, but primarily, shares vest 25% on the first anniversary of the vesting commencement date and then quarterly over three years , with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company. Upon conversion, the vesting terms of the previously issued equity remained consistent. During 2013, the Company recorded compensation expense of $0.1 million and $0.6 million in general and administrative expense and research and development expense, respectively, related to the restricted units. For the year ended December 31, 2014 , the Company recorded compensation expense of $1.2 million and $1.0 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. For the year ended December 31, 2015 , the Company recorded compensation expense of $0.1 million and $1.8 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. In December 2014, 200,000 restricted shares of common stock were issued to The Trustees of the University of Pennsylvania (Penn) in connection with a license agreement (Note 9). The shares are subject to certain milestone-based vesting conditions and had a grant date fair value of $1.5 million . The Company recorded in-process research and development expense of $0.8 million during the year ended December 31, 2014. No milestone vesting conditions were achieved in 2015. At December 31, 2015 , there was $1.8 million of unrecognized compensation expense related to the restricted common shares which is expected to be recognized over a weighted-average period of 1.4 years . The following table summarizes restricted stock activity for the instruments discussed above: Number of units/shares Weighted- average grant date fair value Nonvested shares at December 31, 2013 5,370,000 $ 0.75 Units granted 1,210,667 $ 0.32 Units vested (526,667 ) $ 0.81 Units forfeited (170,000 ) $ 0.84 Conversion to C Corporation and effect of reverse stock split (5,294,713 ) $ — Shares granted 200,000 $ 7.50 Shares vested (210,293 ) $ 5.61 Nonvested shares at December 31, 2014 578,994 $ 4.46 Shares vested (230,439 ) $ 3.87 Nonvested shares at December 31, 2015 348,555 $ 4.83

Stock incentive plans

Stock incentive plans	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans In May 2014, the Company established the 2014 Stock Incentive Plan (the 2014 Plan), which allows for the granting of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards and other stock awards of the Company’s common stock to employees, officers, directors, consultants and advisors. In January 2015, upon the IPO, the 2014 plan was terminated and the 209,500 shares available for future grants under the 2014 Plan were rolled into the 2015 Stock Incentive Plan (the 2015 Plan). In January 2015, the Company established the 2015 Plan, which became effective immediately prior to the closing of the IPO. The 2015 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to employees, officers, directors, consultants and advisors. Under the 2015 Plan, the number of shares of common stock reserved for issuance is the sum of: (1) 1,830,000 plus; (2) the number of shares (up to 2,543,299 shares) equal to the sum of the number of shares of common stock then available for issuance under the 2014 Plan and the number of shares of common stock subject to outstanding awards under the 2014 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,724,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of such fiscal year and an amount determined by the board of directors. As of December 31, 2015 , 970,275 shares were available for future grants under the 2015 Plan. In January 2015, the Company established the 2015 employee stock purchase plan (the 2015 ESPP), which became effective immediately prior to the closing of the IPO. The 2015 ESPP initially will provide participating employees with the opportunity to purchase an aggregate of 220,000 shares of common stock. The number of shares of common stock reserved for issuance under the 2015 ESPP automatically will increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2026, in an amount equal to the lowest of: (1) 440,000 shares of common stock; (2) 1% of the total number of shares of common stock outstanding on the first day of the applicable fiscal year; and (3) an amount determined by the board of directors. No shares were issued under the 2015 ESPP as of December 31, 2015 . The following table summarizes stock option activity: Number of shares Weighted- average exercise price Aggregate intrinsic value(a) Outstanding at December 31, 2013 — $ — Granted 2,357,505 $ 4.48 Forfeited (93,008 ) $ 3.45 Outstanding at December 31, 2014 2,264,497 $ 4.52 Granted 1,031,700 $ 59.80 Exercised (221,825 ) $ 5.03 Forfeited (3,000 ) $ 75.57 Outstanding at December 31, 2015 3,071,372 $ 22.99 Vested at December 31, 2015 599,814 $ 4.31 $ 24,590,030 Vested at December 31, 2015 and expected to vest 3,027,793 $ 22.44 $ 86,791,237 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock. The weighted average remaining contractual term of options outstanding as of December 31, 2015 is 8.8 years. The weighted average remaining contractual term of options exercisable as of December 31, 2015 is 8.6 years. During the year ended December 31, 2014, the Company recorded compensation expense of $0.3 million and $0.5 million in research and development expense and general and administrative expense, respectively, related to stock options. During the year ended December 31, 2015, the Company recorded compensation expense of $3.9 million and $5.9 million in research and development expense and general and administrative expense, respectively, related to stock options. At December 31, 2015 , there was $36.9 million of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted average period of 3.1 years. The weighted average grant date fair value of the options granted in 2014 and 2015 was $3.25 and $40.51 per share, respectively, using the Black-Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2014 2015 Expected volatility 87.6 % 76.8 % Risk-free interest rate 1.90 % 1.67 % Expected term (in years) 6.02 6.10 Expected dividend yield 0.0 % 0.0 % The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2014 — — Shares granted 49,750 $ 60.03 Shares vested (24,650 ) $ 60.03 Nonvested shares at December 31, 2015 25,100 $ 60.03 The Company issued a restricted stock grant to certain employees totaling 49,750 shares in November 2015. The vesting provisions of this grant are based on the acheivement of certain Company milestones. During the year ended December 31, 2015, the Company recorded compensation expense of $1.1 million and $0.6 million in research and development expense and general and administrative expense, respectively, related to restricted common stock. At December 31, 2015 , there was $1.3 million of unrecognized compensation expense related to the performance-based restricted common stock.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies (a) Leases In March 2014, the Company entered into an operating lease for laboratory and office space at its corporate headquarters in Philadelphia, PA, through October 2025. Under this lease, the Company received $8.0 million of tenant improvement allowances during 2014. As of December 31, 2015, $8.1 million is recorded as long-term deferred rent and $0.7 million is recorded as current deferred rent on the accompanying balance sheet. In November 2015, the Company entered into a sublease agreement for approximately 14,000 square feet of additional office space at its corporate headquarters. The sublease will terminate on November 30, 2018. The Company made a $1.2 million upfront payment in connection with the sublease and as of December 31, 2015 the $0.4 million is recorded as prepaid expenses and other current assets and $0.8 million is recorded as other assets on the accompanying balance sheet. In addition, the Company leased approximately 3,400 square feet of office space in Waltham, Massachusetts under a lease that expires in September 2016. Rent expense under these leases was $0.6 million and $1.0 million for the years ended December 31, 2014 and 2015, respectively. Future minimum lease payments under these leases are as follows: Year Ending December 31, 2016 $ 1,997,647 2017 1,941,450 2018 1,961,168 2019 1,691,486 2020 1,733,774 2021 and thereafter 8,792,077 Total $ 18,117,602 b) License agreements See Note 10 for a discussion of the CHOP license agreement. In October 2013, the Company entered into a patent license agreement with Penn for certain intellectual property licenses to be provided by Penn to the Company in the fields of research, development, manufacture and commercialization. On December 31, 2015, the Company amended and restated the patent license agreement with Penn. The license agreement requires the Company to reimburse Penn for the patent costs related to the underlying licensed rights. For the period from March 13, 2013 to December 31, 2013, and for the year ended December 31, 2014 and 2015, the Company recorded $94,501 , $17,840 and $34,873 , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying statements of operations. The Company is obligated to make payments to Penn upon the occurrence of first commercial sale for certain licensed products in both the United States and Europe. The Company must pay a low-single-digit royalty based on net sales of licensed products by territory, which royalties will be reduced if the Company is required to license patents or intellectual property from third parties. In December 2014, the Company entered into a license agreement with Penn for certain intellectual property licenses. The Company issued to Penn 200,000 shares of restricted common stock (Note 7), which are subject to performance-based vesting conditions, in connection with the agreement and is obligated to make milestone payments upon the achievement of certain regulatory milestones up to $5.5 million in the aggregate. Additionally, the Company is obligated to pay Penn single-digit-royalties based on its net sales of licensed products by territory. In October 2013, the Company entered into a license agreement with the University of Iowa Research Foundation (UIRF) for certain intellectual property licenses. The license agreement requires the Company to reimburse UIRF for the patent costs related to the underlying licensed rights. For the period from March 13, 2013 to December 31, 2013, and for the year ended December 31, 2014 and 2015, the Company recorded $0.3 million , $0.3 million and $0.4 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying statements of operations. The Company is obligated to make payments to UIRF upon the occurrence of various development and commercialization milestones. The Company must pay a low-single-digit royalty to UIRF based on net sales of licensed products by territory. In connection with the license agreement, the Company issued 281,854 Series 1 common units (Series 1 Units) to UIRF. The fair value of the units of $0.6 million was recorded as acquired in-process research and development expense in the fourth quarter of 2013.

Related-party transactions

Related-party transactions	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2014 and 2015, CHOP was considered a significant equity holder. In October 2013, the Company entered into technology and license agreements with CHOP for certain commercialization licenses to be provided to the Company in order to develop services, methods and marketable products for commercialization. The license agreement requires the Company to reimburse CHOP for the patent costs related to the underlying licensed rights incurred after the effective date. For the period from inception to December 31, 2013 and for the years ended December 31, 2014 and 2015 , the Company recorded $0.1 million , $0.6 million and $0.7 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying statements of operations. In 2013, the Company entered into a number of services agreements with CHOP. The Master Research Services Agreement provides for certain research, development, and manufacturing services to be provided to the Company by CHOP. A separate Services Agreement provides for clinical, technical, and administrative services to be provided by CHOP to the Company. For the period from inception to December 31, 2013 and for the years ended December 31, 2014 and 2015 , the Company recorded $3.8 million , $6.0 million and $5.2 million , respectively, as research and development expense and for the period from inception to December 31, 2013 and for the year ended December 31, 2014, the Company recorded $31,643 and $49,393 , respectively, as general and administrative expense related to these agreements. As of December 31, 2014 , $0.1 million and $0.9 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of December 31, 2015 , $0.2 million and $1.7 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Collaboration and license agree

Collaboration and license agreements	12 Months Ended
	Dec. 31, 2015
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements In March 2014, the Company entered into an agreement with Genable Technologies Limited (Genable) in which the Company will be the exclusive manufacturer and provide development advice and expertise in the ongoing development of Genable’s lead therapeutic product. Under a license agreement, the Company also granted certain rights to manufacturing patent applications. The Company is eligible to earn development milestone payments and mid-single-digit royalties on future product sales. During the year ended December 31, 2014, the Company received $20,000 for the license and recognized it as revenue. During the year ended December 31, 2015, the Company recognized $0.9 million revenue related to manufacturing of product pursuant to the Genable agreement. In March 2016, the Company acquired Genable (See Note 13). In April 2014, the Company began discussions with a biopharmaceutical company concerning a potential manufacturing technology agreement. The Company received a one-time, nonrefundable payment of $1.0 million to engage in due diligence. In March 2015, the Company concluded discussions on a potential arrangement with the biopharmaceutical company and, as a result, the Company recognized the nonrefundable payment of $1.0 million as revenue during the year ended December 31, 2015 . In December 2014, the Company entered into a global collaboration agreement with Pfizer Inc. (Pfizer), for the development and commercialization of SPK-FIX product candidates for the treatment of hemophilia B. Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024. The Company will be primarily responsible for conducting all research and development activities through completion of Phase 1/2 clinical trials of hemophilia B product candidates. Pfizer and the Company will share development costs incurred under an agreed product development plan for each product candidate with the Company’s share of development costs under the agreement limited to $10.6 million . Following the completion of Phase 1/2 clinical trials, Pfizer will be primarily responsible for development, manufacture, regulatory approval and commercialization, including all costs associated therewith. In connection with this agreement, the Company received a $20.0 million upfront payment for the license in December 2014. As there is no stand-alone value for the license, the Company is recognizing revenue through the estimated completion date of Phase 1/2 clinical trials. In December 2015, the Company earned a $15.0 million milestone payment, which is included in other receivables on the accompanying balance sheet. During the years ended December 31, 2014 and 2015, the Company recognized $0.6 million and $20.2 million of revenue, respectively. As of December 31, 2015 , there is $5.2 million and $9.0 million of current and long term deferred revenue for this payment, respectively. During the years ended December 31, 2014 and 2015, the Company recorded $0.1 million and $1.3 million , respectively, as a reduction to research and development expenses for the reimbursement of costs from Pfizer. The Company is eligible to receive up to an additional $245.0 million in aggregate milestone payments, $125.0 million of which relate to potential development, regulatory and commercial milestones for the first product candidate to achieve each milestone and $120.0 million of which relate to potential regulatory milestones for additional product candidates. In addition, the Company is entitled to receive royalties calculated as a low-teen percentage of net sales of licensed products. The royalties may be subject to certain reductions, including for a specified portion of royalty payments that Pfizer may become required to pay under any third-party license agreements, subject to a minimum royalty. Under the agreement, the Company remains solely responsible for the payment of license payments payable by the Company under specified license agreements. The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product; or (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements. In April 2015, the Company entered into a research, license and option agreement with Clearside Biomedical, Inc. (Clearside) under which the Company acquired exclusive rights to license Clearside’s microinjector technology and the option to further develop and commercialize gene therapy products delivered using the Clearside technology. Under the agreement, the companies will explore the feasibility of using Clearside’s microinjector technology to deliver viral vectors to the choroid and the retina through the suprachoroidal space. In connection with this agreement, the Company made an upfront payment of $0.5 million for services to be rendered in the development of licensed products. During the year ended December 31, 2015, the Company recorded $0.5 million as research and development expense related to the upfront payment.

Income taxes Income taxes

Income taxes Income taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income taxes	Income taxes A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: December 31, 2014 2015 Federal income tax expense at statutory rate 34.0 % 34.0 % Loss prior to C corporation conversion (7.1 ) — Permanent differences (0.6 ) (2.7 ) Tax credits 7.3 13.0 Change in valuation allowance (33.6 ) (44.3 ) Effective income tax rate — % — % Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets are comprised of the following: December 31, 2014 2015 Deferred tax assets (liabilities): U.S. net operating loss carryforwards $ 5,133,763 $ 11,385,094 Tax credit carryforwards 2,623,967 11,694,170 Stock based compensation 916,695 3,363,805 Deferred rent 3,709,831 3,581,023 Deferred revenue 625,987 6,376,558 Accruals and other 378,976 2,024,542 Fixed Assets (3,433,053 ) (4,598,986 ) Total net deferred tax assets 9,956,166 33,826,206 Less valuation allowance (9,956,166 ) (33,826,206 ) Net deferred taxes $ — $ — As of December 31, 2015, the Company had U.S. federal net operating loss carryforwards of 25.5 million , which may be available to offset future income tax liabilities and will expire in 2034. As of December 31, 2015, the Company also had U.S. state net operating loss carryforwards of $24.5 million which may be available to offset future income tax liabilities and will expire in 2034 . The Company has recorded a full valuation allowance against its net deferred tax assets as of December 31, 2015 and 2014, respectively, because the Company has determined that is it more likely than not that these assets will not be fully realized due to historic net operating losses incurred. The Company experienced a net change in valuation allowance of $10.0 million and $23.4 million in the years ended December 31, 2014 and 2015, respectively. As of December 31, 2015, the Company had federal research and development and orphan drug tax credit carryforwards of $0.7 million and $11.0 million , respectively, available to reduce future tax liabilities which expire in 2034 . Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss 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 financing 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 will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2015, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statements of operations. For the year ended December 31, 2015, the Company generated research credits but has not conducted a study to document the qualified activities. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position for these years. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, 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.

Subsequent events

Subsequent events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events [Text Block]	Subsequent Events On March 7, 2016, the Company acquired Dublin, Ireland-based Genable, a private gene therapy company with which the Company has collaborated since 2014 in the development of Genable's therapeutic program targeting a form of inherited retinal disease (IRD). With the acquisition, the Company acquires RhoNova™, a potential treatment targeting rhodopsin-linked autosomal dominant retinitis pigmentosa (RHO-adRP), an IRD that routinely leads to visual impairment and in the most severe cases to blindness. The consideration paid to Genable shareholders consisted of $6 million in cash and 265,000 shares of the Company's common stock.

Selected quarterly financial da

Selected quarterly financial data (unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Selected quarterly financial data	Selected quarterly financial data (unaudited) The following table contains quarterly financial information for 2014 and 2015. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2014 (1) First Quarter Second Quarter Third Quarter Fourth Quarter Total Revenues $ 20,000 $ — $ — $ 613,932 $ 633,932 Research and development 3,387,733 2,129,573 4,652,056 6,181,643 16,351,005 Acquired in-process research and development — — — 750,000 750,000 General and administrative 1,016,123 2,038,909 2,106,877 2,701,347 7,863,256 Total operating expenses 4,403,856 4,168,482 6,758,933 9,632,990 24,964,261 Loss from operations (4,383,856 ) (4,168,482 ) (6,758,933 ) (9,019,058 ) (24,330,329 ) Net loss $ (4,383,856 ) $ (4,168,384 ) $ (6,756,924 ) $ (9,015,645 ) $ (24,324,809 ) Basic and diluted net loss per common unit $ (0.86 ) $ (0.79 ) $ (1.22 ) $ (1.74 ) $ (4.64 ) 2015 (1) First Quarter Second Quarter Third Quarter Fourth Quarter Total Revenues $ 2,274,467 $ 1,288,629 $ 1,302,789 $ 17,197,789 $ 22,063,674 Research and development 8,334,108 9,343,972 11,796,455 16,554,779 46,029,314 General and administrative 3,684,880 6,333,123 6,461,675 6,872,493 23,352,171 Total operating expenses 12,018,988 15,677,095 18,258,130 23,427,272 69,381,485 Loss from operations (9,744,521 ) (14,388,466 ) (16,955,341 ) (6,229,483 ) (47,317,811 ) Net loss $ (9,733,507 ) $ (14,336,842 ) $ (16,900,560 ) $ (6,154,869 ) $ (47,125,778 ) Basic and diluted net loss per common share $ (0.58 ) $ (0.60 ) $ (0.70 ) $ (0.25 ) $ (2.10 ) (1) The sum of the quarterly per share amounts may not equal per share amounts reported for the year due to rounding.

Summary of significant accoun21

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Use of estimates	Use of estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates.
Fair value of financial instruments	Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, other receivables and accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of December 31, 2014 and 2015 consisted primarily of money market funds.
Property and equipment	Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded 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, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment 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 undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. No impairment charges have been recorded since inception.
Research and development and in-process research and development	Research and development and in-process research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use.
Income tax	Income taxes From inception through May 1, 2014, the Company was a Delaware LLC for federal and state tax purposes and, therefore, all items of income or loss through May 1, 2014 flowed through to the members of the LLC. Effective May 2, 2014, the Company converted from an LLC to a C corporation for federal and state income tax purposes. Accordingly, prior to the conversion to a C corporation, the Company did not record deferred tax assets or liabilities or have any net operating loss carryforwards. The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes, if any, is applied during the years in which temporary differences are expected to be settled and is reflected in the financial statements in the period of enactment. The measurement of deferred tax assets is reduced, if necessary, if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2015, the Company has concluded that a full valuation allowance is necessary for its deferred tax assets.
Revenue recognition	Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Topic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: • the milestone is commensurate with either: (1) the performance required to achieve the milestone or (2) the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; • the milestone relates solely to past performance; and • the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. To date, the Company has not generated any revenues from the commercial sale of products.
Share-based compensation and fair value of stock	Stock-based compensation and fair value of stock 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 and modifications to existing stock options, to be recognized in the statements of operations based on their fair values. The Company uses the Black-Scholes option pricing model to determine the fair value of options granted. The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and directors with service-based vesting conditions is recognized on a straight-line basis based on the 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, using the accelerated attribution method. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through share-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed over the vesting period. The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option pricing model, which requires the input of and subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate, and (iv) 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 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 will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. 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. The Company also is required to estimate forfeitures at the time of grant, and revise those estimates in subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate pre-vesting option forfeitures and records stock-based compensation expense only for those awards that are expected to vest. 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. Stock-based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest. 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 and restricted stock award 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 over the contractual life.
Recapitalization	Recapitalization On January 16, 2015, the Company effected a reverse stock split of the Company’s common stock at a ratio of one share for every five shares previously held. All common stock share and common stock per share data included in these financial statements reflect the reverse stock split.
Net loss per common share	Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented, the outstanding shares of convertible preferred stock, unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same.
Deferred rent	Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property, which was May 1, 2014 for the Company’s corporate headquarters. Tenant improvement allowances from the lessor are included in the accompanying balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of December 31, 2015 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received.
Recent accounting pronouncements	Recent Accounting Pronouncements In November 2015, the FASB issued ASU 2015-17, “Balance Sheet Classification of Deferred Taxes”, which requires that deferred tax assets and liabilities be classified as noncurrent in a classified statement of financial position. This update is effective for financial statements issued for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years, and may be applied either prospectively to all deferred tax assets and liabilities or retrospectively to all periods presented. The Company elected to early-adopt this update on a retrospective basis. The adoption of this update had no effect on the Company's results of operations. In February 2016, the FASB issued ASU 2016-02, “Leases.” ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. The updated guidance is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company has not evaluated the impact of the updated guidance on the Company's financial statements.

Summary of significant accoun22

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding	The following potentially dilutive securities have been excluded from the computations of diluted weighted average units/shares outstanding as of December 31, 2014 and 2015 as they would be anti-dilutive: Period from March 13, 2013 (inception) to December 31, December 31, 2013 2014 2015 Convertible preferred units 5,000,000 — — Unvested restricted common units 5,370,000 — — Convertible preferred shares — 10,037,255 — Unvested restricted common shares — 578,994 373,655 Options issued and outstanding — 2,264,497 3,071,372

Fair value of financial instr23

Fair value of financial instruments (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	The Company has classified assets and liabilities measured at fair value on a recurring basis as follows: Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2014: Assets: Money market funds (included in cash and cash equivalents) $ 74,025,841 — — At December 31, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 293,530,590 — —

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	Property and equipment consist of the following: December 31, 2014 2015 Laboratory equipment $ 2,961,494 $ 4,415,121 Computer and software 52,710 158,280 Furniture and fixtures 290,818 470,151 Office equipment 59,635 72,308 Leasehold improvements 5,113,269 11,274,721 Construction in progress 4,366,236 2,511,636 Property and equipment, gross 12,844,162 18,902,217 Less accumulated depreciation and amortization (169,790 ) (1,902,772 ) Property and equipment, net $ 12,674,372 $ 16,999,445

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following: December 31, 2014 2015 Compensation and benefits $ 1,385,013 $ 4,880,239 Consulting and professional fees 1,327,942 432,346 Research and development 247,448 978,156 Other 202,751 238,522 $ 3,163,154 $ 6,529,263

Stockholders' equity (Tables)

Stockholders' equity (Tables)	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Summary of Restricted Stock Activity	The following table summarizes restricted stock activity for the instruments discussed above: Number of units/shares Weighted- average grant date fair value Nonvested shares at December 31, 2013 5,370,000 $ 0.75 Units granted 1,210,667 $ 0.32 Units vested (526,667 ) $ 0.81 Units forfeited (170,000 ) $ 0.84 Conversion to C Corporation and effect of reverse stock split (5,294,713 ) $ — Shares granted 200,000 $ 7.50 Shares vested (210,293 ) $ 5.61 Nonvested shares at December 31, 2014 578,994 $ 4.46 Shares vested (230,439 ) $ 3.87 Nonvested shares at December 31, 2015 348,555 $ 4.83 The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2014 — — Shares granted 49,750 $ 60.03 Shares vested (24,650 ) $ 60.03 Nonvested shares at December 31, 2015 25,100 $ 60.03

Stock incentive plans (Tables)

Stock incentive plans (Tables)	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes stock option activity: Number of shares Weighted- average exercise price Aggregate intrinsic value(a) Outstanding at December 31, 2013 — $ — Granted 2,357,505 $ 4.48 Forfeited (93,008 ) $ 3.45 Outstanding at December 31, 2014 2,264,497 $ 4.52 Granted 1,031,700 $ 59.80 Exercised (221,825 ) $ 5.03 Forfeited (3,000 ) $ 75.57 Outstanding at December 31, 2015 3,071,372 $ 22.99 Vested at December 31, 2015 599,814 $ 4.31 $ 24,590,030 Vested at December 31, 2015 and expected to vest 3,027,793 $ 22.44 $ 86,791,237 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock.
Summary of Weighted-Average Assumptions	The weighted average grant date fair value of the options granted in 2014 and 2015 was $3.25 and $40.51 per share, respectively, using the Black-Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2014 2015 Expected volatility 87.6 % 76.8 % Risk-free interest rate 1.90 % 1.67 % Expected term (in years) 6.02 6.10 Expected dividend yield 0.0 % 0.0 %
Summary of Restricted Common Stock Activity	The following table summarizes restricted stock activity for the instruments discussed above: Number of units/shares Weighted- average grant date fair value Nonvested shares at December 31, 2013 5,370,000 $ 0.75 Units granted 1,210,667 $ 0.32 Units vested (526,667 ) $ 0.81 Units forfeited (170,000 ) $ 0.84 Conversion to C Corporation and effect of reverse stock split (5,294,713 ) $ — Shares granted 200,000 $ 7.50 Shares vested (210,293 ) $ 5.61 Nonvested shares at December 31, 2014 578,994 $ 4.46 Shares vested (230,439 ) $ 3.87 Nonvested shares at December 31, 2015 348,555 $ 4.83 The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2014 — — Shares granted 49,750 $ 60.03 Shares vested (24,650 ) $ 60.03 Nonvested shares at December 31, 2015 25,100 $ 60.03

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	Future minimum lease payments under these leases are as follows: Year Ending December 31, 2016 $ 1,997,647 2017 1,941,450 2018 1,961,168 2019 1,691,486 2020 1,733,774 2021 and thereafter 8,792,077 Total $ 18,117,602

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: December 31, 2014 2015 Federal income tax expense at statutory rate 34.0 % 34.0 % Loss prior to C corporation conversion (7.1 ) — Permanent differences (0.6 ) (2.7 ) Tax credits 7.3 13.0 Change in valuation allowance (33.6 ) (44.3 ) Effective income tax rate — % — %
Schedule of Deferred Tax Assets	The significant components of the Company’s deferred tax assets are comprised of the following: December 31, 2014 2015 Deferred tax assets (liabilities): U.S. net operating loss carryforwards $ 5,133,763 $ 11,385,094 Tax credit carryforwards 2,623,967 11,694,170 Stock based compensation 916,695 3,363,805 Deferred rent 3,709,831 3,581,023 Deferred revenue 625,987 6,376,558 Accruals and other 378,976 2,024,542 Fixed Assets (3,433,053 ) (4,598,986 ) Total net deferred tax assets 9,956,166 33,826,206 Less valuation allowance (9,956,166 ) (33,826,206 ) Net deferred taxes $ — $ —

Selected quarterly financial 30

Selected quarterly financial data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly financial information	The following table contains quarterly financial information for 2014 and 2015. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2014 (1) First Quarter Second Quarter Third Quarter Fourth Quarter Total Revenues $ 20,000 $ — $ — $ 613,932 $ 633,932 Research and development 3,387,733 2,129,573 4,652,056 6,181,643 16,351,005 Acquired in-process research and development — — — 750,000 750,000 General and administrative 1,016,123 2,038,909 2,106,877 2,701,347 7,863,256 Total operating expenses 4,403,856 4,168,482 6,758,933 9,632,990 24,964,261 Loss from operations (4,383,856 ) (4,168,482 ) (6,758,933 ) (9,019,058 ) (24,330,329 ) Net loss $ (4,383,856 ) $ (4,168,384 ) $ (6,756,924 ) $ (9,015,645 ) $ (24,324,809 ) Basic and diluted net loss per common unit $ (0.86 ) $ (0.79 ) $ (1.22 ) $ (1.74 ) $ (4.64 ) 2015 (1) First Quarter Second Quarter Third Quarter Fourth Quarter Total Revenues $ 2,274,467 $ 1,288,629 $ 1,302,789 $ 17,197,789 $ 22,063,674 Research and development 8,334,108 9,343,972 11,796,455 16,554,779 46,029,314 General and administrative 3,684,880 6,333,123 6,461,675 6,872,493 23,352,171 Total operating expenses 12,018,988 15,677,095 18,258,130 23,427,272 69,381,485 Loss from operations (9,744,521 ) (14,388,466 ) (16,955,341 ) (6,229,483 ) (47,317,811 ) Net loss $ (9,733,507 ) $ (14,336,842 ) $ (16,900,560 ) $ (6,154,869 ) $ (47,125,778 ) Basic and diluted net loss per common share $ (0.58 ) $ (0.60 ) $ (0.70 ) $ (0.25 ) $ (2.10 )

Background - Additional Informa

Background - Additional Information (Detail)	Dec. 28, 2015USD ($)$ / sharesshares	Feb. 04, 2015USD ($)$ / sharesshares	Dec. 31, 2013USD ($)	Dec. 31, 2015USD ($)Segmentshares	Dec. 31, 2014USD ($)
Class of Stock [Line Items]
Common stock, shares issued during IPO	2,266,995
Number of operating segments | Segment				1
Proceeds from follow-on public offering | $	$ 106,500,000
Aggregate gross proceeds from IPO | $		$ 185,200,000	$ 0	$ 270,400,216	$ 0
Net proceeds from IPO | $	$ 99,400,000	$ 168,900,000
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO				10,316,995
Stock issuance price per share | $ / shares	$ 47
Preferred stock, shares issued upon conversion		10,200,050
Initial Public Offering [Member]
Class of Stock [Line Items]
IPO completion date				Feb. 4, 2015
Initial Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO		8,050,000
Stock issuance price per share | $ / shares		$ 23

Development-Stage Risks - Addit

Development-Stage Risks - Additional Information (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Development-stage Risk [Abstract]
Retained earnings (accumulated deficit)	$ 128,728,384	$ 81,602,606

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Additional Information (Detail)	Jan. 16, 2015	Dec. 31, 2013USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Property, Plant and Equipment [Line Items]
Impairment charges of long-lived assets		$ 0	$ 0	$ 0
Reverse stock split, description			One share for every five shares previously held.
Conversion ratio of common stock	0.2
Computers [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life			3 years
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life			3 years
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life			5 years
Furniture & Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life			7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives			Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding (Detail) - shares	9 Months Ended	10 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2015
Convertible Preferred Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding		5,000,000
Unvested Restricted Common Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding		5,370,000
Convertible Preferred Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	10,037,255	0	0
Unvested Restricted Common Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	578,994	0	373,655
Options Issued and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	2,264,497	0	3,071,372

Fair Value of Financial Instr35

Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - Money Market Funds [Member] - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 293,530,590	$ 74,025,841
Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 0	$ 0

Property and equipment, net (De

Property and equipment, net (Details) - USD ($)	10 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 18,902,217	$ 12,844,162
Less accumulated depreciation and amortization		(1,902,772)	(169,790)
Property and equipment, net		16,999,445	12,674,372
Depreciation and amortization expense	$ 0	1,732,983	169,790
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		4,415,121	2,961,494
Computer and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		158,280	52,710
Furniture & Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		470,151	290,818
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		72,308	59,635
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		11,274,721	5,113,269
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 2,511,636	$ 4,366,236

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Compensation and benefits	$ 4,880,239	$ 1,385,013
Consulting and professional fees	432,346	1,327,942
Research and development	978,156	247,448
Other	238,522	202,751
Accrued expenses	$ 6,529,263	$ 3,163,154

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Feb. 04, 2015	Dec. 31, 2014	May. 31, 2014	Oct. 31, 2013	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 28, 2015	May. 01, 2014
Class of Stock [Line Items]
Common stock, shares authorized		150,000,000			150,000,000	150,000,000			150,000,000						150,000,000	150,000,000
Preferred stock, shares authorized		5,000,000			5,000,000	5,000,000			5,000,000						5,000,000	5,000,000
Preferred stock, shares issued		0			0				0						0	0
Proceeds from issuance of Series B convertible preferred stock, net														$ 0	$ 0	$ 72,437,203
Preferred stock, value		$ 0			$ 0				$ 0						0	0
Accumulated dividends	$ 1,300,000
Grant date fair value of restricted stock issued in connection with license agreement														50,000,000		750,000
Research and development					$ 16,554,779	$ 11,796,455	$ 9,343,972	$ 8,334,108	$ 6,181,643	$ 4,652,056	$ 2,129,573	$ 3,387,733		4,897,152	$ 46,029,314	$ 16,351,005
Common Stock [Member]
Class of Stock [Line Items]
Stock issuance price per share																	$ 47
Preferred stock, shares issued upon conversion	10,200,050
Preferred shares converted into number of common stock	10,200,050
Units, vesting description															The vesting terms of the common stock issued upon conversion of the Series 2 Units and Series 3 Units vary, but primarily, shares vest 25% on the first anniversary of the vesting commencement date and then quarterly over three years, with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company.
Issuance of restricted stock in connection with license agreement (in shares)																200,000
Grant date fair value of restricted stock issued in connection with license agreement																$ 200
Common Stock [Member] | Tranche One [Member]
Class of Stock [Line Items]
Percentage of vesting of units on first anniversary															25.00%
Common Stock [Member] | Tranche Two [Member]
Class of Stock [Line Items]
Units, vesting period															3 years
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion															Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized		5,000,000							5,000,000							5,000,000
Preferred stock, shares issued		5,000,000							5,000,000							5,000,000
Convertible preferred stock terms of conversion															Upon conversion of the Company into a C corporation in May 2014, each outstanding Series A Unit converted into one share of Series A Stock.
Preferred stock, shares issued upon conversion					1										1
Preferred stock, value		$ 10,000,000			$ 0				$ 10,000,000						$ 0	$ 10,000,000
Preferred stock dividend rate			8.00%
Series A Convertible Preferred Stock [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion			0.2
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized		45,186,334							45,186,334							45,186,334
Preferred stock, shares issued		45,186,334	45,186,334						45,186,334							45,186,334
Preferred stock, value		$ 72,437,203	$ 72,400,000		0				$ 72,437,203						$ 0	$ 72,437,203
Minimum percentage of approval by outstanding preferred share holders for preferred stock conversion to common stock			87.50%
Preferred stock dividend rate			8.00%
Series B Convertible Preferred Stock [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion			0.2
Series 1 Unit [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion															Each Series A Unit was convertible into one Series 1 Unit (subject to certain antidilution adjustments) at any time at the option of the holder.
Preferred stock, shares issued upon conversion				1
Series 1 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common units, shares issued upon conversion																		0.2
Series 2 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common units, shares issued upon conversion																		0.2
Series 3 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common units, shares issued upon conversion																		0.03883773
CHOP [Member] | Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				5,000,000
Stock issuance price per share				$ 2
Proceeds from issuance of Series B convertible preferred stock, net				$ 10,000,000
License Agreement [Member] | University of Pennsylvania [Member]
Class of Stock [Line Items]
Issuance of restricted stock in connection with license agreement (in shares)		200,000
Restricted Shares [Member]
Class of Stock [Line Items]
Unrecognized compensation expense related to restricted common shares					$ 1,800,000										$ 1,800,000
Unrecognized compensation expense related to restricted common shares expected to be recognized, weighted-average period															1 year 4 months 18 days
Restricted Shares [Member] | General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense													$ 1,200,000	100,000	$ 100,000
Restricted Shares [Member] | Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense													$ 1,000,000	$ 600,000	$ 1,800,000
Restricted Shares [Member] | License Agreement [Member] | University of Pennsylvania [Member]
Class of Stock [Line Items]
Issuance of restricted stock in connection with license agreement (in shares)		200,000
Grant date fair value of restricted stock issued in connection with license agreement		$ 1,500,000
Research and development																$ 800,000

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Restricted Stock Activity (Detail) - Restricted Shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Number of shares (in shares):
Nonvested beginning balance (in shares)	578,994	5,370,000
Shares granted (in shares)		200,000
Shares vested (in shares)	(230,439)	(210,293)
Nonvested ending balance (in shares)	348,555	578,994
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in usd per share)	$ 4.46	$ 0.75
Shares granted (in usd per share)		7.50
Shares vested (in usd per share)	5.61	3.87
Nonvested shares ending balance (in usd per share)	$ 4.83	$ 4.46
Member Units [Member]
Number of shares (in shares):
Shares granted (in shares)		1,210,667
Shares vested (in shares)		(526,667)
Units forfeited, in shares		(170,000)
Conversion to C Corporation and effect of reverse stock split, in shares		(5,294,713)
Weighted-average grant date fair value (in dollars per share):
Shares granted (in usd per share)		$ 0.32
Shares vested (in usd per share)		0.81
Units forfeited, in dollars per share		0.84
Conversion to C Corporation and effect of reverse stock split, in dollars per share		$ 0

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining contractual term of options outstanding		8 years 9 months
Weighted average remaining contractual term of options exercisable		8 years 7 months 10 days
Unrecognized compensation expense related to stock options		$ 36.9
Weighted average grant date fair value of options		$ 40.51	$ 3.25
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period recognition for unrecognized compensation expense		3 years 1 month 18 days
Employee Stock Option [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 3.9	$ 0.3
Employee Stock Option [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 5.9	$ 0.5
Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted (in shares)		49,750
Unrecognized compensation expense related to the performance-based restricted common stock		$ 1.3
Performance-Based Restricted Common Stock [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		1.1
Performance-Based Restricted Common Stock [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 0.6
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants	209,500	970,275
Common stock reserved for issuance	1,830,000
Maximum shares authorized	2,543,299
Shares of common stock	1,724,000
Percentage of common stock outstanding	4.00%
Stock incentive plans description		Under the 2015 Plan, the number of shares of common stock reserved for issuance is the sum of: (1) 1,830,000 plus; 2) the number of shares (up to 2,543,299 shares) equal to the sum of the number of shares of common stock then available for issuance under the 2014 Plan and the number of shares of common stock subject to outstanding awards under the 2014 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,724,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of such fiscal year and an amount determined by the board of directors.
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	220,000
Shares of common stock	440,000
Percentage of common stock outstanding	1.00%
Stock incentive plans description		The number of shares of common stock reserved for issuance under the 2015 ESPP automatically will increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2026, in an amount equal to the lowest of: (1) 440,000 shares of common stock; (2) 1% of the total number of shares of common stock outstanding on the first day of the applicable fiscal year; and (3) an amount determined by the board of directors. No shares were issued under the 2015 ESPP plan as of June 30, 2015
Number of shares issued		0

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Number of shares
Outstanding beginning balance (in shares)	2,264,497	0
Granted (in shares)	1,031,700	2,357,505
Exercised (in shares)	(221,825)
Forfeited (in shares)	(3,000)	(93,008)
Outstanding ending balance (in shares)	3,071,372	2,264,497
Vested, outstanding (in shares)	599,814
Vested and expected to vest, outstanding (in shares)	3,027,793
Weighted-average exercise price
Outstanding beginning balance (in usd per share)	$ 4.52	$ 0
Granted (in usd per share)	59.80	4.48
Exercised (in usd per share)	5.03
Forfeited (in usd per share)	75.57	3.45
Outstanding ending balance (in usd per share)	22.99	$ 4.52
Vested, outstanding (in usd per share)	4.31
Vested and expected to vest, outstanding (in usd per share)	$ 22.44
Vested, outstanding, aggregate intrinsic value	$ 24,590,030
Vested and expected to vest, outstanding, aggregate intrinsic value	$ 86,791,237

Stock Incentive Plans - Summa42

Stock Incentive Plans - Summary of Weighted-Average Assumptions (Detail) - Employee Stock Option [Member]	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	76.80%	87.60%
Risk-free interest rate	1.67%	1.90%
Expected term	6 years 1 month 6 days	6 years 7 days
Expected dividend yield	0.00%	0.00%

Stock Incentive Plans - Summa43

Stock Incentive Plans - Summary of Restricted Common Stock Activity (Detail) (Details) - Performance-Based Restricted Common Stock [Member]	12 Months Ended
	Dec. 31, 2015$ / sharesshares
Number of shares (in shares):
Nonvested beginning balance (in shares) | shares	0
Shares granted (in shares) | shares	49,750
Shares vested (in shares) | shares	(24,650)
Nonvested ending balance (in shares) | shares	25,100
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in usd per share) | $ / shares	$ 0
Shares granted (in usd per share) | $ / shares	60.03
Shares vested (in usd per share) | $ / shares	60.03
Nonvested shares ending balance (in usd per share) | $ / shares	$ 60.03

Commitments and Contingencies44

Commitments and Contingencies (Details)	1 Months Ended			3 Months Ended									10 Months Ended	12 Months Ended
	Nov. 30, 2015USD ($)ft²	Dec. 31, 2014USD ($)shares	Oct. 31, 2013shares	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Sep. 30, 2014USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Property Subject to or Available for Operating Lease [Line Items]
Long-term deferred rent		$ 8,618,489		$ 8,084,509				$ 8,618,489						$ 8,084,509	$ 8,618,489
Current portion of deferred rent		$ 0		715,959				0						715,959	0
Operating leases, rent expense														1,000,000	600,000
General and administrative				6,872,493	$ 6,461,675	$ 6,333,123	$ 3,684,880	2,701,347	$ 2,106,877	$ 2,038,909	$ 1,016,123		$ 2,380,645	23,352,171	7,863,256
Issuance of common units in connection with license and technology agreements													50,000,000		750,000
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,016				1,997,647										1,997,647
2,017				1,941,450										1,941,450
2,018				1,961,168										1,961,168
2,019				1,691,486										1,691,486
2,020				1,733,774										1,733,774
2021 and thereafter				8,792,077										8,792,077
Total				18,117,602										18,117,602
University of Pennsylvania [Member] | License Agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
General and administrative													94,501	34,873	17,840
Issuance of restricted stock in connection with license agreement (in shares) | shares		200,000
University of Iowa Research Foundation [Member] | License Agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
General and administrative													$ 300,000	400,000	300,000
Issuance of restricted stock in connection with license agreement (in shares) | shares			281,854
Issuance of common units in connection with license and technology agreements												$ 600,000
Lease of Laboratory and Office Space [Member]
Property Subject to or Available for Operating Lease [Line Items]
Allowance for leasehold improvements															8,000,000
Lease of Laboratory and Office Space [Member] | Corporate HQ [Member]
Property Subject to or Available for Operating Lease [Line Items]
Long-term deferred rent				8,100,000										8,100,000
Current portion of deferred rent				700,000										700,000
Lease of Office Space [Member] | Office Space in Waltham, Mass [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of real estate property | ft²	3,400
Sublease of Office Space [Member]
Property Subject to or Available for Operating Lease [Line Items]
Payments for rent	$ 1,200,000
Sublease of Office Space [Member] | Prepaid Expenses and Other Current Assets [Member]
Property Subject to or Available for Operating Lease [Line Items]
Prepaid rent				$ 400,000										$ 400,000
Sublease of Office Space [Member] | Corporate HQ [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of real estate property | ft²	14,000
Maximum [Member] | Milestone Payments [Member] | University of Pennsylvania [Member] | License Agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
Agreement obligation, milestone payments		$ 5,500,000						$ 5,500,000							$ 5,500,000

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
General and administrative related to agreements	$ 6,872,493	$ 6,461,675	$ 6,333,123	$ 3,684,880	$ 2,701,347	$ 2,106,877	$ 2,038,909	$ 1,016,123	$ 2,380,645	$ 23,352,171	$ 7,863,256
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	200,000				100,000					200,000	100,000
Accounts payable due to related party	$ 1,700,000				$ 900,000					1,700,000	900,000
CHOP [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements									3,800,000	5,200,000	6,000,000
General and administrative related to agreements									31,643		49,393
CHOP [Member] | General and Administrative Expense [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs									$ 100,000	$ 700,000	$ 600,000

Collaboration and License Agr46

Collaboration and License Agreements - Additional Information (Detail) - USD ($)	Apr. 30, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Apr. 30, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Current portion of deferred revenue		$ 10,014,377	$ 5,182,835	$ 10,014,377
Long term deferred revenue		10,767,414	9,034,559	10,767,414
Genable License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Licenses revenue				20,000
Revenue recognized			900,000
Potential Manufacturing Technology Agreement with Unnamed Biopharmaceutical Company [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Licenses revenue			1,000,000
Nonrefundable payment received					$ 1,000,000
Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Licenses revenue			20,200,000	600,000
Current portion of deferred revenue			$ 5,200,000
Collaborative arrangement rights and obligations			Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024.
Collaborative arrangement, maximum development costs to be incurred				10,600,000
Upfront payment received for license		$ 20,000,000
Milestone payments earned included in other receivables			$ 15,000,000
Long term deferred revenue			9,000,000
Revenue recognition from aggregate milestone payments			$ 245,000,000
Collaboration and license agreement expiration description			The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product or; (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements.
Collaboration agreement expiration, period			15 years
First Product Candidate [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition from aggregate milestone payments			$ 125,000,000
Additional Product Candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition from aggregate milestone payments			120,000,000
Clearside Biomedical [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current	$ 500,000
Research and development expense			500,000
Research and Development Expense [Member] | Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursement revenue			$ 1,300,000	$ 100,000

Income Taxes - Effective Rate R

Income Taxes - Effective Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	34.00%	34.00%
Loss prior to C corporation conversion	(0.00%)	(7.10%)
Permanent differences	(2.70%)	(0.60%)
Tax credits	13.00%	7.30%
Change in valuation allowance	(44.30%)	(33.60%)
Effective income tax rate	0.00%	0.00%

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Tax Credit Carryforward [Line Items]
U.S. net operating loss carryforwards	$ 11,385,094	$ 5,133,763
Tax credit carryforwards	11,694,170	2,623,967
Stock based compensation	3,363,805	916,695
Deferred rent	3,581,023	3,709,831
Deferred revenue	6,376,558	625,987
Accruals and other	2,024,542	378,976
Fixed Assets	(4,598,986)	(3,433,053)
Total net deferred tax assets	33,826,206	9,956,166
Less valuation allowance	(33,826,206)	(9,956,166)
Net deferred taxes	0	0
Valuation allowance changes	23,400,000	$ 10,000,000
Research Tax Credit Carryforward [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	700,000
Orphan Drug Tax Credit Carryforward [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	11,000,000
Domestic Tax Authority [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards	25,500,000
State and Local Jurisdiction [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards	$ 24,500,000

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event [Member] - Genable [Member] $ in Millions	Mar. 07, 2016USD ($)shares
Business Acquisition [Line Items]
Payments to acquire business | $	$ 6
Common Stock [Member]
Business Acquisition [Line Items]
Business acquisition, equity issued (in shares) | shares	265,000

Selected quarterly financial 50

Selected quarterly financial data (unaudited) (Details) - USD ($)	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 17,197,789	$ 1,302,789	$ 1,288,629	$ 2,274,467	$ 613,932	$ 0	$ 0	$ 20,000	$ 0	$ 22,063,674	$ 633,932
Research and development	16,554,779	11,796,455	9,343,972	8,334,108	6,181,643	4,652,056	2,129,573	3,387,733	4,897,152	46,029,314	16,351,005
Acquired in-process research and development					750,000	0	0	0	50,000,000	0	750,000
General and administrative	6,872,493	6,461,675	6,333,123	3,684,880	2,701,347	2,106,877	2,038,909	1,016,123	2,380,645	23,352,171	7,863,256
Total operating expenses	23,427,272	18,258,130	15,677,095	12,018,988	9,632,990	6,758,933	4,168,482	4,403,856	57,277,797	69,381,485	24,964,261
Loss from operations	(6,229,483)	(16,955,341)	(14,388,466)	(9,744,521)	(9,019,058)	(6,758,933)	(4,168,482)	(4,383,856)	(57,277,797)	(47,317,811)	(24,330,329)
Net loss	$ (6,154,869)	$ (16,900,560)	$ (14,336,842)	$ (9,733,507)	$ (9,015,645)	$ (6,756,924)	$ (4,168,384)	$ (4,383,856)	$ (57,277,797)	$ (47,125,778)	$ (24,324,809)
Basic and diluted net loss per common share/unit (in usd per share/unit)	$ (0.25)	$ (0.70)	$ (0.60)	$ (0.58)	$ (1.74)	$ (1.22)	$ (0.79)	$ (0.86)	$ (8.44)	$ (2.10)	$ (4.64)

Uncategorized Items - once-2015

Label	Element	Value
Limited Liability Company (LLC) Members' Equity, Unit-based Compensation	us-gaap_LimitedLiabilityCompanyLLCMembersEquityUnitBasedCompensation	$ 646,585
Stock Issued During Period, Value, New Issues	us-gaap_StockIssuedDuringPeriodValueNewIssues	10,000,000
Member Units [Member]
Limited Liability Company (LLC) Members' Equity, Unit-based Compensation	us-gaap_LimitedLiabilityCompanyLLCMembersEquityUnitBasedCompensation	646,585
Stock Issued During Period, Value, Purchase of Assets	us-gaap_StockIssuedDuringPeriodValuePurchaseOfAssets	$ 50,000,000
Stock Issued During Period, Shares, Purchase of Assets	us-gaap_StockIssuedDuringPeriodSharesPurchaseOfAssets	25,000,000
Stock Issued During Period, Shares, New Issues	us-gaap_StockIssuedDuringPeriodSharesNewIssues	5,870,000
Retained Earnings [Member]
Net Income (Loss) Attributable to Parent	us-gaap_NetIncomeLoss	$ (57,277,797)
Series A Preferred Stock [Member] | Member Units [Member]
Stock Issued During Period, Value, New Issues	us-gaap_StockIssuedDuringPeriodValueNewIssues	$ 10,000,000
Stock Issued During Period, Shares, New Issues	us-gaap_StockIssuedDuringPeriodSharesNewIssues	5,000,000