Spark Therapeutics (ONCE) 10-K 28 Feb 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Feb. 22, 2017	Jun. 30, 2016
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	Large Accelerated Filer
Document Type	10-K
Trading Symbol	ONCE
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus (Q1,Q2,Q3,FY)	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		31,044,367
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,112,326,819

Consolidated balance sheets

Consolidated balance sheets - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 58,923,097	$ 293,530,590
Marketable securities	237,242,655	0
Other receivables	16,780,917	16,944,568
Prepaid expenses	1,647,008	1,132,626
Total current assets	314,593,677	311,607,784
Marketable securities	21,900,129	0
Property and equipment, net	19,794,306	16,999,445
Acquired in-process research and development	15,490,000	0
Goodwill	1,160,104	0
Other assets	924,579	1,165,285
Total assets	373,862,795	329,772,514
Current liabilities:
Accounts payable	9,928,737	9,687,594
Accrued expenses	13,826,920	6,529,263
Current portion of long-term debt	302,013	0
Current portion of deferred rent	771,196	715,959
Current portion of deferred revenue	5,168,674	5,182,835
Total current liabilities	29,997,540	22,115,651
Long-term debt	1,224,003	0
Long-term deferred rent	7,498,419	8,084,509
Long-term deferred revenue	3,865,885	9,034,559
Deferred tax liability	1,000,235	0
Total liabilities	43,586,082	39,234,719
Stockholders’ equity:
Preferred stock, $0.001 par value. Authorized, 5,000,000 shares; no shares issued or outstanding	0	0
Common stock, $0.001 par value. Authorized, 150,000,000 shares; 27,082,493 shares issued and 27,073,287 outstanding at December 31, 2015; 30,873,430 shares issued and 30,864,224 outstanding at December 31, 2016	30,874	27,083
Additional paid-in capital	583,973,682	419,791,732
Accumulated other comprehensive loss	(794,296)	0
Treasury stock, at cost 9,206 shares at December 31, 2015 and 2016	(552,636)	(552,636)
Accumulated deficit	(252,380,911)	(128,728,384)
Total stockholders’ equity	330,276,713	290,537,795
Total liabilities and stockholders’ equity	$ 373,862,795	$ 329,772,514

Consolidated balance sheets (un

Consolidated balance sheets (unaudited) (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Preferred stock , par value (in usd per share)	$ 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 (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	30,873,430	27,082,493
Common stock, shares outstanding	30,864,224	27,073,287
Treasury stock, shares	9,206	9,206

Consolidated statements of oper

Consolidated statements of operations and other comprehensive income (loss) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenues	$ 20,182,835	$ 22,063,674	$ 633,932
Operating expenses:
Research and development	86,379,405	46,029,314	16,351,005
Acquired in-process research and development	11,132,146	0	750,000
General and administrative	48,070,317	23,352,171	7,863,256
Total operating expenses	145,581,868	69,381,485	24,964,261
Loss from operations	(125,399,033)	(47,317,811)	(24,330,329)
Interest income, net	1,746,506	192,033	5,520
Net loss	(123,652,527)	(47,125,778)	(24,324,809)
Preferred stock dividends	0	(634,794)	(707,342)
Net loss applicable to common stockholders	$ (123,652,527)	$ (47,760,572)	$ (25,032,151)
Basic and diluted net loss per common share (in usd per share)	$ (4.29)	$ (2.10)	$ (4.64)
Weighted average basic and diluted common shares outstanding (in shares)	28,804,133	22,710,105	5,397,599
Other comprehensive income (loss):
Unrealized loss on available-for-sale securities	$ 7,421	$ 0	$ 0
Foreign exchange translation adjustment	(801,717)	0	0
Total comprehensive loss	$ (124,446,823)	$ (47,760,572)	$ (25,032,151)

Consolidated statement of stock

Consolidated 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]	AOCI Attributable to Parent [Member]	Accumulated Deficit [Member]
Beginning balance, common units at Dec. 31, 2013	$ 3,368,788	$ 50,646,585	$ 10,000,000	$ 0		$ 0			$ 0			$ 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
Beginning balance, common stock (in shares) at Dec. 31, 2013				0		0			0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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 common stock in connection with license agreement	750,000								$ 200			749,800
Issuance of restricted stock in connection with license agreement (in shares)									200,000
Issuance of convertible preferred stock/units	72,437,203					$ 72,437,203
Issuance of convertible preferred stock/units (in shares/units)						45,186,334
Issuance of restricted stock (in shares)		1,040,667	0
Stock-based compensation expense	2,974,538											2,974,538
Foreign exchange translation adjustment	0
Unrealized loss on available-for-sale securities	0
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		$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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 convertible preferred stock/units	268,321,943								$ 10,317			268,311,626
Issuance of convertible preferred stock/units (in shares/units)									10,316,995
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 (in shares)									49,750
Stock-based compensation expense	13,378,594											13,378,594
Purchase of common stock in treasury	(552,636)							$ (552,636)
Exercise of stock options	$ 1,116,043								$ 222			1,115,821
Exercise of stock options (in shares)	221,825								221,825
Foreign exchange translation adjustment	$ 0
Unrealized loss on available-for-sale securities	0
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			$ 0	(128,728,384)
Ending balance, common stock (in shares) at Dec. 31, 2015				0		0		(9,206)	27,082,493
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of convertible preferred stock/units	127,566,064								$ 3,025			127,563,039
Issuance of convertible preferred stock/units (in shares/units)									3,025,000
Stock-based compensation expense	24,537,436											24,537,436
Purchase of common stock in treasury (in shares)								(9,206)
Exercise of stock options	$ 2,591,922								$ 455			2,591,467
Exercise of stock options (in shares)	455,138								455,138
Restricted stock canceled	$ 0								$ (2)			2
Restricted stock canceled (in shares)									(2,213)
Issuance of restricted stock, not vested	0								$ 40			(40)
Issuance of restricted stock, not vested (in shares)									40,000
Purchase of common stock under ESPP	339,869								$ 8			339,861
Purchase of common stock under ESPP (in shares)									8,012
Issuance of stock for acquisition	9,150,450								$ 265			9,150,185
Issuance of stock for purchase of Genable (in shares)									265,000
Foreign exchange translation adjustment	(801,717)														(801,717)
Unrealized loss on available-for-sale securities	7,421														7,421
Net loss	(123,652,527)															(123,652,527)
Ending balance, common stock at Dec. 31, 2016	$ 330,276,713							$ (552,636)	$ 30,874			$ 583,973,682			$ (794,296)	$ (252,380,911)
Ending balance, common stock (in shares) at Dec. 31, 2016								(9,206)	30,873,430

Consolidated statement of stoc6

Consolidated statement of stockholders' equity (Parenthetical)	12 Months Ended
	Dec. 31, 2014USD ($)
Preferred Stock [Member] | Series B Convertible Preferred Stock [Member]
Transaction costs	$ 312,795

Consolidated statements of cash

Consolidated statements of cash flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (123,652,527)	$ (47,125,778)	$ (24,324,809)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Noncash rent expense	(530,853)	181,979	617,114
Depreciation and amortization expense	3,634,349	1,732,983	169,790
Loss on disposal of property and equipment	101,490	0	0
Acquired in-process research and development	11,132,146	0	750,000
Stock-based compensation expense	24,537,436	13,572,503	2,974,538
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(264,230)	(1,627,602)	(670,309)
Other receivables	(234,803)	(16,700,175)	(144,393)
Accounts payable and accrued expenses	10,018,084	9,052,260	2,331,282
Deferred rent	0	0	7,901,375
Deferred revenue	(5,182,835)	(6,564,397)	20,781,791
Net cash provided by (used in) operating activities	(80,441,743)	(47,478,227)	10,386,379
Cash flows from investing activities:
Purchases of marketable securities	(279,944,501)	0	0
Proceeds from maturities of marketable securities	20,000,000	0	0
Purchase of acquired in-process research and development	(11,132,146)	0	0
Payment for acquisition, net of cash acquired	(5,911,243)	0	0
Purchases of property and equipment	(8,532,397)	(4,521,769)	(11,696,962)
Net cash used in investing activities	(285,520,287)	(4,521,769)	(11,696,962)
Cash flows from financing activities:
Proceeds from issuance of Series A convertible preferred units	0	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	2,591,922	1,116,043	0
Proceeds from public offerings of common stock, net	126,908,733	270,400,216	0
Proceeds from issuance of common stock under ESPP	339,869	0	0
Proceeds from long-term debt	1,550,610	0	0
Payments on long-term debt	(24,594)	0	0
Net cash provided by financing activities	131,366,540	270,963,623	75,877,546
Effect of exchange rate changes on cash and cash equivalents	(12,003)	0	0
Net increase (decrease) in cash and cash equivalents	(234,607,493)	218,963,627	74,566,963
Cash and cash equivalents, beginning of period	293,530,590	74,566,963	0
Cash and cash equivalents, end of period	58,923,097	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	$ 681,790	$ 2,683,487	$ 1,147,200

Background

Background	12 Months Ended
	Dec. 31, 2016
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) Public Offerings On February 4, 2015 , the Company closed its initial public offering, or 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. On June 20, 2016, the Company completed a follow-on public offering, having sold 3,025,000 shares of common stock at an offering price of $45 .00 per share, for aggregate gross proceeds of $136.1 million . The Company received net proceeds from the public offering of $127.6 million , after deducting underwriting discounts and commissions and other offering expenses.

Development-stage risks

Development-stage risks	12 Months Ended
	Dec. 31, 2016
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 $252.4 million at December 31, 2016 . 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, 2016
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Use of estimates The preparation of consolidated 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 consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates. (b) Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Spark Therapeutics, Inc, Spark Therapeutics Ireland Limited and Spark Therapeutics England Limited. All intercompany balances and transactions have been eliminated in consolidation. (c) Fair value of financial instruments Management believes that the carrying amounts of the Company’s consolidated 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. Management believes the carrying value of debt approximates fair value as the interest rates are reflective of the rate the Company could obtain on debt with similar terms and conditions. (d) 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, 2015 and 2016 consisted primarily of money market funds. (e) Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2016, the balance in the Company’s accumulated other comprehensive loss included activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the maturity of available-for-sale securities during the year ended December 31, 2016 and, as a result, the Company did not reclassify any amount out of accumulated other comprehensive loss for the same period. In addition, as part of the license and stock purchase agreements entered into with Selecta Biosciences, Inc. (Selecta) (note 14), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value. (f) 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. (g) Acquired in-process research and development and goodwill The acquired in-process research and development (IPR&D) asset is an indefinite-lived intangible asset and is assessed for impairment annually or more frequently if impairment indicators exist. If the associated research and development effort is abandoned, the related assets will be written off and the Company will record a noncash impairment loss on its consolidated statement of operations. For those compounds that reach commercialization, the IPR&D assets will be amortized over their estimated useful lives. The impairment test for indefinite-lived intangible assets is a one-step test, which compares the fair value of the intangible asset to its carrying value. If the carrying value exceeds its fair value, an impairment loss is recognized in an amount equal to the excess. Based on accounting standards, it is required that these assets be assessed at least annually for impairment unless a triggering event occurs between annual assessments which would then require an assessment in the period which a triggering event occurred. The Company performed its annual impairment test as of October 1, 2016 and there was no impairment to acquired in-process research and development in 2016. Goodwill represents the excess of purchase price over the fair value of net assets acquired by the Company. Goodwill is not amortized, but assessed for impairment on an annual basis or more frequently if impairment indicators exist. The Company operates as one reporting unit. The Company has the option to perform a qualitative assessment of goodwill prior to completing the two-step method described below to determine whether or not it is more likely than not that the fair value of its reporting units is less than its carrying amount, including goodwill and other intangible assets. If the Company concludes that this is the case, it must perform the two-step process. The first step compares a reporting unit’s fair value to its carrying amount to identify potential goodwill impairment. If the carrying amount of a reporting unit exceeds the reporting unit’s fair value, the second step of the impairment test must be completed to measure the amount of the reporting unit’s goodwill impairment loss, if any. Step two requires an assignment of the reporting unit’s fair value to the reporting unit’s assets and liabilities to determine the implied fair value of the reporting unit’s goodwill. The implied fair value of the reporting unit’s goodwill is then compared with the carrying amount of the reporting unit’s goodwill to determine the goodwill impairment loss to be recognized, if any. The Company performs its annual goodwill impairment test as of October 1st. The Company performed a qualitative assessment in 2016 and determined that there was no impairment to goodwill for the year ended December 31, 2016. (h) 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. (i) 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 and 2016, the Company has concluded that a full valuation allowance is necessary for its deferred tax assets. (j) 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. (k) 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 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. 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 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 substantial 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 accounts for forfeitures when they occur. Stock-based compensation expense recognized in the financial statements is based on actual awards forfeited. 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. (l) 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. (m) 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 , 2015 and 2016 as they would be anti-dilutive: December 31, 2014 2015 2016 Convertible preferred shares 10,037,255 — — Unvested restricted common shares 578,994 373,655 251,809 Options issued and outstanding 2,264,497 3,071,372 4,181,993 Amounts in the table above reflect the common stock equivalents of the noted instruments. (n) 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. 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 and 2016 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received. (o) Other comprehensive loss The Company follows the provisions of FASB ASC Topic 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive income is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. The Company recorded gains and losses related to changes in available for sale securities and foreign currency translation. (p) Recent Accounting Pronouncements In February 2016, the FASB issued Accounting Standards Update, (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 consolidated financial statements. In May 2014, the FASB issued updated guidance regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The update provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The Company currently is evaluating the effect that this guidance may have on its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 14. In March 2016, the FASB issued ASU 2016-09, " Improvements to Employee Share-Based Payment Accounting. " ASU 2016-09 intends to reduce the cost and complexity of accounting for share-based payments. The updated guidance is effective for interim and annual periods beginning after December 15, 2016, and early adoption is permitted. The Company early adopted the guidance as of March 31, 2016 and the impact was immaterial to the Company's consolidated financial statements. In January 2016, the FASB issued ASU 2016-01, " Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities ". ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to either measure equity investments without readily determinable fair values at fair value or at cost adjusted for changes in observable prices minus impairment. Changes in measurement under either alternative will be recognized in net income. The updated guidance is effective for interim periods and annual periods beginning after December 31, 2017. A cumulative-effect adjustment to the balance sheet will be recorded as of the beginning of the fiscal year of adoption. The Company has not evaluated the impact of the updated guidance on the Company’s consolidated financial statements but expects to see an increase in the volatility of net loss as the volatility currently recorded in other comprehensive income related to changes in the fair market value of the available-for-sale equity investments will be reflected in net loss after adoption. In January 2017, the FASB issued ASU 2017-01, “ Clarifying the Definition of a Business ”. ASU 2017-01 clarified the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions of assets or businesses. The guidance is effective for interim and annual periods beginning after December 31, 2017, and early adoption is permitted. The Company elected to early adopt this guidance in the current period and has applied it to its evaluation of the license and stock agreement entered into with Selecta (note 14).

Marketable securities

Marketable securities	12 Months Ended
	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the available-for-sale securities held at December 31, 2016 : Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2016 U.S. government agency $ 133,690,267 $ 10,907 $ (85,714 ) $ 133,615,460 Corporate securities $ 126,253,903 $ — $ (726,579 ) $ 125,527,324 No available-for-sale securities held as of December 31, 2016 had remaining maturities greater than two years .

Business Acquisition of Genable

Business Acquisition of Genable	12 Months Ended
	Dec. 31, 2016
Business Combinations [Abstract]
Business Acquisition of Genable	Business acquisition of Genable On March 7, 2016, the Company acquired Genable Technologies, Ltd. (Genable), an Ireland-based private gene therapy company with which the Company had collaborated since 2014 in the development of Genable's therapeutic program targeting a genetic inherited retinal disease (IRD). With the acquisition, the Company acquired RhoNova™, a potential gene therapy 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 by the Company to Genable shareholders consisted of $6.1 million in cash and 265,000 shares of the Company's common stock with a fair value of $9.2 million , for total consideration of $15.3 million . In connection with the acquistion, a receivable due from Genable also was settled on the date of acquisition for $0.5 million . The Company incurred acquisition-related costs of approximately $0.3 million , which are included in general and administrative expenses on the consolidated statement of operations for the year ended December 31, 2016 . The Company has accounted for the acquisition as a business combination under the acquisition method of accounting. The Company has allocated the purchase price for the purchase of Genable based upon the estimated fair value of net assets acquired and liabilities assumed at the date of acquisition. The completion and filing of federal and state tax returns for the purchased entity may result in adjustments to the carrying value of assets and liabilities. Recognition and measurement of assets acquired and liabilities assumed The following table summarizes the fair values of the tangible and intangible assets acquired and liabilities assumed at the acquisition date: Cash acquired $ 196,307 Other current assets 102,506 Acquired in-process research and development 15,490,000 Goodwill 1,160,104 Total assets assumed 16,948,917 Other non-current liabilities 254,753 Deferred tax liability 1,000,235 Total liabilities assumed 1,254,988 Total allocation of purchase price $ 15,693,929 In the fourth quarter of 2016, goodwill and deferred tax liability were reduced by $936,015 , representing the value of net operating loss carryforwards available for use in Ireland. Acquired in-process research and development The Company’s allocation of purchase price to acquired in-process research and development was $15.5 million . The estimated fair value of the in-process research and development was determined using the “income approach,” which is a valuation technique that provides an estimate of the fair value of an asset based on market participant expectations of the cash flows an asset would generate over its remaining useful life. Some of the more significant assumptions inherent in the development of those asset valuations include the estimated net cash flows for each year for the asset or product (including net revenues, cost of sales, research and development costs, selling and marketing costs and working capital/asset contributory asset charges), the appropriate discount rate to select in order to measure the risk inherent in each future cash flow stream, the assessment of the asset’s life cycle, the potential regulatory and commercial success risks, competitive trends impacting the asset cash flow stream as well as other factors. No assurances can be given that the underlying assumptions used to prepare the discounted cash flow analysis will not change. For these and other reasons, actual results may vary significantly from estimated results.

Fair value of financial instrum

Fair value of financial instruments	12 Months Ended
	Dec. 31, 2016
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, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 293,530,590 — — At December 31, 2016: Assets: Money market funds (included in cash and cash equivalents) $ 53,452,424 — — Corporate securities (included in cash and cash equivalents) $ 5,029,838 Marketable securities - U.S. government agencies $ 133,615,460 — — Marketable securities - corporate securities $ 122,144,692 3,382,632

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Property and equipment, net	Property and equipment, net Property and equipment consist of the following: December 31, 2015 2016 Laboratory equipment $ 4,415,121 $ 5,833,616 Computer and software 158,280 530,663 Furniture and fixtures 470,151 470,151 Office equipment 72,308 537,756 Leasehold improvements 11,274,721 16,981,989 Construction in progress 2,511,636 967,945 Property and equipment, gross 18,902,217 25,322,120 Less accumulated depreciation and amortization (1,902,772 ) (5,527,814 ) Property and equipment, net $ 16,999,445 $ 19,794,306 Depreciation and amortization expense was $1.7 million and $3.6 million for the years ended December 31, 2015 and 2016 , respectively.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2016
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following: December 31, 2015 2016 Compensation and benefits $ 4,880,239 $ 9,613,275 Consulting and professional fees 432,346 995,614 Research and development 978,156 2,717,777 Other 238,522 500,254 $ 6,529,263 $ 13,826,920

Debt

Debt	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Debt	Debt In August 2016, the Company executed an agreement with the Commonwealth of Pennsylvania to fund machinery and equipment and other assets purchased (MELF Loan) in the amount of $1.6 million . Borrowings under the MELF Loan are secured by equipment, as defined in the loan agreement. Under the terms of the MELF Loan, the Company has a five year period of monthly payments of $28,928 of principal and interest at an annual interest rate of 3.25% . The Company recorded interest expense of $14,533 related to the MELF Loan for the year ended December 31, 2016.

Stockholders' equity

Stockholders' equity	12 Months Ended
	Dec. 31, 2016
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 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 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 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. In connection with the conversion to a C corporation, the units converted to common stock. The vesting terms of the restricted common stock varied, 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. 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. For the year ended December 31, 2016 , the Company recorded compensation expense of $0.1 million and $1.3 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 12). 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 or 2016 . At December 31, 2016 , there was $0.6 million of unrecognized compensation expense related to the restricted common shares which is expected to be recognized over a weighted-average period of 0.6 years . The following table summarizes restricted stock activity for the instruments discussed above: Number of shares Weighted- average grant date fair value 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 Shares canceled (1,213 ) $ 1.15 Shares vested (172,466 ) $ 3.90 Nonvested shares at December 31, 2016 174,876 $ 5.78

Stock incentive plans

Stock incentive plans	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans In January 2015, the Company established the 2015 Stock Incentive Plan (the 2015 Plan), which became effective immediately prior to the closing of the IPO. The 2015 Plan replaced the 2014 Stock Incentive Plan (the 2014 Plan) and the 209,500 shares available for future grants under the 2014 Plan were rolled into the 2015 Plan. 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, canceled, 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. In January 2016, the number of shares of common stock authorized for issuance under the 2015 Plan automatically increased, pursuant to the terms of the 2015 Plan, by 1,083,313 shares. As of December 31, 2016 , 430,542 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 provided 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. In January 2016, the number of shares of common stock authorized for issuance under the 2015 ESPP automatically increased, pursuant to the terms of the Plan, by 270,828 shares. As of December 31, 2016 , 482,816 shares of common stock are available to provide participating employees with the opportunity to purchase shares of common stock. The following table summarizes stock option activity: Number of shares Weighted- average exercise price Aggregate intrinsic value(a) 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 Granted 1,722,950 $ 43.16 Exercised (455,138 ) $ 5.69 Forfeited (157,191 ) $ 21.17 Outstanding at December 31, 2016 4,181,993 $ 33.25 Vested at December 31, 2016 1,159,926 $ 24.58 $ 35,495,985 Vested at December 31, 2016 and expected to vest 4,056,533 $ 33.25 $ 83,508,162 (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 that were in the money at December 31, 2016. The weighted average remaining contractual term of options outstanding as of December 31, 2016 is 8.3 years. The weighted average remaining contractual term of options exercisable as of December 31, 2016 is 7.5 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. During the year ended December 31, 2016, the Company recorded compensation expense of $9.2 million and $13.2 million in research and development expense and general and administrative expense, respectively, related to stock options. At December 31, 2016 , there was $64.3 million of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted average period of 2.9 years. The intrinsic value of options exercised during the years ended December 31, 2015 and 2016 was $10.4 million and $21.1 million , respectively. The weighted average grant date fair value of the options granted in 2014, 2015 and 2016 was $3.25 , $40.51 and $28.56 per share, respectively, using the Black-Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2014 2015 2016 Expected volatility 87.6 % 76.8 % 75.2 % Risk-free interest rate 1.90 % 1.67 % 1.71 % Expected term (in years) 6.02 6.10 5.95 Expected dividend yield 0.0 % 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 Shares granted 59,500 $ 44.35 Shares cancelled (1,000 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at December 31, 2016 76,933 $ 49.90 Included in the above table are non-vested restricted stock grants to certain employees and consultants totaling 43,600 shares for which the vesting provisions are based on achievement 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. During the year ended December 31, 2016, the Company recorded compensation expense of $0.2 million and $0.4 million in research and development expense and general and administrative expense, respectively, related to restricted common stock. At December 31, 2016 , there was $3.3 million of unrecognized compensation expense related to the restricted common stock, which is expected to be recognized over a weighted average period of 1.9 years. ESPP stock activity: No shares were issued to employees during 2015 and 8,012 shares were issued under the ESPP during the year ended December 31, 2016. During the year ended December 31, 2016 , the Company recorded compensation expense of $0.1 million and $0.1 million in research and development expense and general and administrative expense, respectively, related to the 2015 ESPP. At December 31, 2016 , there was no unrecognized compensation expense related to the ESPP.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2016
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. 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. In February 2016, the Company leased 6,500 square feet of additional office space in Philadelphia under a lease that expires in June 2021. In November 2016, the Company entered in an additional lease agreement for approximately 49,000 square feet of office space in Philadelphia that will commence on April 1, 2017 and terminate on March 31, 2027. Under this lease, the Company will receive $3.2 million of tenant improvements allowances during 2017. In addition, the Company leases approximately 5,400 square feet of office space in Waltham, Massachusetts under a lease that expires in March 2022. As of December 31, 2016 , $7.5 million is recorded as long-term deferred rent and $0.8 million is recorded as current deferred rent on the accompanying consolidated balance sheet. Rent expense under these leases was $1.0 million and $1.9 million for the years ended December 31, 2015 and 2016 , respectively. Future minimum lease payments under these leases are as follows: Year Ending December 31, 2017 $ 4,410,126 2018 4,495,760 2019 4,293,484 2020 4,405,160 2021 4,398,678 2022 and thereafter 19,592,238 Total $ 41,595,446 (b) License agreements See note 13 for a discussion of the CHOP license agreement. In October 2013, the Company entered into a patent license agreement with the University of Pennsylvania, (Penn), as amended, for certain intellectual property licenses to be provided by Penn to the Company in the fields of research, development, manufacture and commercialization. The license agreement requires the Company to reimburse Penn for the patent costs related to the underlying licensed rights. 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 10), 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. 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.

Related-party transactions

Related-party transactions	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2015 and 2016 , 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 years ended December 31, 2014 , 2015 and 2016 , the Company recorded $0.6 million , $0.7 million and $0.9 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 years ended December 31, 2014 , 2015 and 2016 , the Company recorded $6.0 million , $5.2 million and $7.4 million , respectively, as research and development expense. For the year ended December 31, 2014, the Company recorded $49,393 as general and administrative expense related to these agreements. For the years ended December 31, 2015 and 2016, the company did not record any general and administrative expense related to these agreements. 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. As of December 31, 2016 , $1.1 million and $0.9 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, 2016
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements In March 2014, the Company entered into an agreement with Genable in which the Company was 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. 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 (note 6). 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, 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 of both 2015 and 2016, the Company earned a $15.0 million milestone payment, which is included in other receivables on the accompanying balance sheets. During the years ended December 31, 2014, 2015 and 2016, the Company recognized $0.6 million , $20.2 million and $20.2 million of revenue, respectively. As of December 31, 2016 , there is $5.2 million and $3.9 million of current and long term deferred revenue, respectively, related to the upfront payment on the consolidated balance sheet. During the years ended December 31, 2014 , 2015 and 2016 , the Company recorded $0.1 million , $1.3 million and $2.4 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 $230.0 million in aggregate milestone payments, $110.0 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. In December 2016, the Company entered into a License and Option Agreement (License Agreement) with Selecta that provides the Company with exclusive worldwide rights to Selecta’s proprietary Synthetic Vaccine Particles (SVP™) platform technology for co-administration with gene therapy targets. Under the terms of the License Agreement, Selecta has granted the Company certain exclusive, worldwide, royalty-bearing licenses to Selecta’s intellectual property and know-how relating to its SVP technology to research, develop and commercialize gene therapies for factor VIII, an essential blood clotting protein relevant to the treatment of hemophilia A, which is the initial target under the license. In addition, for a specified period of time, the Company may exercise options to research, develop and commercialize gene therapies utilizing the SVP technology for up to four additional targets, subject to the Company’s payment of the applicable option exercise fee, in a range of $1.4 million to $2.0 million depending on the incidence of the applicable indication, to Selecta in each case. Each party is responsible for its own costs and expenses incurred in connection with its respective activities under the License Agreement, except that the Company has agreed to reimburse Selecta in respect of full-time equivalents and out-of-pocket costs incurred in performing certain tasks or assistance specifically requested by the Company. Selecta retains the responsibility to manufacture the Company’s preclinical, clinical and commercial requirements for the SVP technology, subject to the terms of the License Agreement. In connection with the execution of the License Agreement the Company paid Selecta an upfront payment of $10.0 million . Additional payments in the aggregate of $5.0 million are due within 12 months. On a target-by-target basis, the Company will be responsible to pay up to an aggregate of $430.0 million in milestone payments for each target, with up to $65.0 million being based on the Company’s achievement of specified development and regulatory milestones and up to $365.0 million for commercial milestones, as well as tiered royalties on global net sales at percentages ranging from mid-single to low-double digits. For a period of 3 years , the Company has the right to fund up to 50% of any development or regulatory milestone payable to Selecta by issuing to Selecta shares of the Company’s common stock having a fair market value equal to the percentage of such development or regulatory milestone, as applicable. The License Agreement will continue on a country-by-country and product-by-product basis until the expiration of the Company's royalty payment obligations with respect to such product in such country unless earlier terminated by the parties. The License Agreement may be terminated by the Company for convenience upon 90 days ’ notice and the Company will not be required to make any payments. Either party may terminate the License Agreement on a target-by-target basis for material breach with respect to such target. In connection with the License Agreement, the Company entered into a Stock Purchase Agreement (SPA) with Selecta pursuant to which the Company purchased 197,238 unregistered shares of Selecta’s common stock for $5.0 million . The fair value of these shares is classified as available for sale securities at December 31, 2016. Under the terms of the Stock Purchase Agreement the Company will purchase additional shares of Selecta’s common stock for an aggregate of $10.0 million in 2017, unless the License Agreement is terminated. The Company accounted for the License Agreement and SPA as a basket transaction and allocated the $15.0 million in cash payments to the shares of Selecta’s common stock and the License Agreement in the amounts of $3.9 million and $11.1 million , respectively. The Company calculated the $3.9 million allocated for the Selecta shares acquired based on the closing market price on the date of purchase. The remaining $11.1 million was allocated to the License Agreement and expensed as acquired in-process research and development as the Company determined there was no alternative future use.

Income taxes Income taxes

Income taxes Income taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income taxes	Income taxes A reconciliation of income tax benefit computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: Year Ended December 31, 2014 2015 2016 Federal income tax benefit at statutory rate 34.0 % 34.0 % 34.0 % Loss prior to C corporation conversion (7.1 ) — — State and local tax, net of federal benefit 7.3 8.0 10.4 Permanent differences (0.6 ) (2.7 ) 3.0 Tax credits 7.3 13.1 9.1 Change in valuation allowance (40.9 ) (52.4 ) (56.5 ) 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, 2015 2016 Deferred tax assets (liabilities): Net operating loss carryforwards $ 11,385,094 $ 55,710,846 Tax credit carryforwards 11,694,170 28,327,477 Stock based compensation 3,363,805 10,342,547 Deferred rent 3,581,023 3,356,430 Deferred revenue 6,376,558 4,057,998 Accruals and other 2,024,542 9,262,530 Total deferred tax assets 38,425,192 111,057,828 Less valuation allowance (33,826,206 ) (107,616,937 ) Net deferred tax assets $ 4,598,986 $ 3,440,891 Intangible assets $ — $ (1,936,250 ) Fixed assets (4,598,986 ) (2,504,876 ) Total deferred tax liabilities $ (4,598,986 ) $ (4,441,126 ) Net deferred tax liability $ — $ (1,000,235 ) As of December 31, 2016 , the Company had U.S. federal net operating loss carryforwards of $121.9 million , which may be available to offset future income tax liabilities and will expire beginning in 2034. As of December 31, 2016 , the Company also had U.S. state net operating loss carryforwards of $119.7 million which may be available to offset future income tax liabilities and will expire beginning in 2034 . The Company has recorded a full valuation allowance against its net deferred tax assets as of December 31, 2016 and 2015 , 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 $23.4 million and $73.8 million in the years ended December 31, 2015 and 2016 , respectively. As of December 31, 2016 , the Company had federal research and development and orphan drug tax credit carryforwards of $1.6 million and $26.7 million , respectively, available to reduce future tax liabilities which expire beginning 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 files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal, state, and foreign jurisdictions, where applicable. The company’s tax years are still open under status from 2013 to present. All open years maybe be examined to the extent that tax credit or net operating loss carryforward are used in future periods. The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2016 , the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s consolidated statements of operations. For the year ended December 31, 2016, 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.

Selected quarterly financial da

Selected quarterly financial data (unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Selected quarterly financial data (unaudited)	Selected quarterly financial data (unaudited) The following table contains quarterly financial information for 2015 and 2016 . 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. 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 ) 2016 (1) First Quarter Second Quarter Third Quarter Fourth Quarter Total Revenues $ 1,288,628 $ 1,288,629 $ 1,302,789 $ 16,302,789 $ 20,182,835 Research and development 18,251,900 19,621,536 22,384,109 26,121,860 86,379,405 Acquired in-process research and development — — — 11,132,146 11,132,146 General and administrative 8,873,861 10,676,752 12,049,954 16,469,750 48,070,317 Total operating expenses 27,125,761 30,298,288 34,434,063 53,723,756 145,581,868 Loss from operations (25,837,133 ) (29,009,659 ) (33,131,274 ) (37,420,967 ) (125,399,033 ) Net loss $ (25,576,711 ) $ (28,676,115 ) $ (32,562,407 ) $ (36,837,294 ) $ (123,652,527 ) Basic and diluted net loss per common share $ (0.95 ) $ (1.04 ) $ (1.07 ) $ (1.21 ) $ (4.29 ) (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 accoun24

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Use of estimates	Use of estimates The preparation of consolidated 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 consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates.
Principles of consolidation	Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Spark Therapeutics, Inc, Spark Therapeutics Ireland Limited and Spark Therapeutics England Limited. All intercompany balances and transactions have been eliminated in consolidation.
Fair value of financial instruments	Fair value of financial instruments Management believes that the carrying amounts of the Company’s consolidated 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. Management believes the carrying value of debt approximates fair value as the interest rates are reflective of the rate the Company could obtain on debt with similar terms and conditions.
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, 2015 and 2016 consisted primarily of money market funds.
Marketable securities	Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2016, the balance in the Company’s accumulated other comprehensive loss included activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the maturity of available-for-sale securities during the year ended December 31, 2016 and, as a result, the Company did not reclassify any amount out of accumulated other comprehensive loss for the same period. In addition, as part of the license and stock purchase agreements entered into with Selecta Biosciences, Inc. (Selecta) (note 14), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value.
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.
Acquired in-process research and development and goodwill	Acquired in-process research and development and goodwill The acquired in-process research and development (IPR&D) asset is an indefinite-lived intangible asset and is assessed for impairment annually or more frequently if impairment indicators exist. If the associated research and development effort is abandoned, the related assets will be written off and the Company will record a noncash impairment loss on its consolidated statement of operations. For those compounds that reach commercialization, the IPR&D assets will be amortized over their estimated useful lives. The impairment test for indefinite-lived intangible assets is a one-step test, which compares the fair value of the intangible asset to its carrying value. If the carrying value exceeds its fair value, an impairment loss is recognized in an amount equal to the excess. Based on accounting standards, it is required that these assets be assessed at least annually for impairment unless a triggering event occurs between annual assessments which would then require an assessment in the period which a triggering event occurred. The Company performed its annual impairment test as of October 1, 2016 and there was no impairment to acquired in-process research and development in 2016. Goodwill represents the excess of purchase price over the fair value of net assets acquired by the Company. Goodwill is not amortized, but assessed for impairment on an annual basis or more frequently if impairment indicators exist. The Company operates as one reporting unit. The Company has the option to perform a qualitative assessment of goodwill prior to completing the two-step method described below to determine whether or not it is more likely than not that the fair value of its reporting units is less than its carrying amount, including goodwill and other intangible assets. If the Company concludes that this is the case, it must perform the two-step process. The first step compares a reporting unit’s fair value to its carrying amount to identify potential goodwill impairment. If the carrying amount of a reporting unit exceeds the reporting unit’s fair value, the second step of the impairment test must be completed to measure the amount of the reporting unit’s goodwill impairment loss, if any. Step two requires an assignment of the reporting unit’s fair value to the reporting unit’s assets and liabilities to determine the implied fair value of the reporting unit’s goodwill. The implied fair value of the reporting unit’s goodwill is then compared with the carrying amount of the reporting unit’s goodwill to determine the goodwill impairment loss to be recognized, if any. The Company performs its annual goodwill impairment test as of October 1st. The Company performed a qualitative assessment in 2016 and determined that there was no impairment to goodwill for the year ended December 31, 2016.
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 and 2016, 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 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. 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 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 substantial 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 accounts for forfeitures when they occur. Stock-based compensation expense recognized in the financial statements is based on actual awards forfeited. 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. 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 and 2016 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received
Other comprehensive loss	Other comprehensive loss The Company follows the provisions of FASB ASC Topic 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive income is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. The Company recorded gains and losses related to changes in available for sale securities and foreign currency translation.
Recent accounting pronouncements	Recent Accounting Pronouncements In February 2016, the FASB issued Accounting Standards Update, (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 consolidated financial statements. In May 2014, the FASB issued updated guidance regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The update provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The Company currently is evaluating the effect that this guidance may have on its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 14. In March 2016, the FASB issued ASU 2016-09, " Improvements to Employee Share-Based Payment Accounting. " ASU 2016-09 intends to reduce the cost and complexity of accounting for share-based payments. The updated guidance is effective for interim and annual periods beginning after December 15, 2016, and early adoption is permitted. The Company early adopted the guidance as of March 31, 2016 and the impact was immaterial to the Company's consolidated financial statements. In January 2016, the FASB issued ASU 2016-01, " Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities ". ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to either measure equity investments without readily determinable fair values at fair value or at cost adjusted for changes in observable prices minus impairment. Changes in measurement under either alternative will be recognized in net income. The updated guidance is effective for interim periods and annual periods beginning after December 31, 2017. A cumulative-effect adjustment to the balance sheet will be recorded as of the beginning of the fiscal year of adoption. The Company has not evaluated the impact of the updated guidance on the Company’s consolidated financial statements but expects to see an increase in the volatility of net loss as the volatility currently recorded in other comprehensive income related to changes in the fair market value of the available-for-sale equity investments will be reflected in net loss after adoption. In January 2017, the FASB issued ASU 2017-01, “ Clarifying the Definition of a Business ”. ASU 2017-01 clarified the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions of assets or businesses. The guidance is effective for interim and annual periods beginning after December 31, 2017, and early adoption is permitted. The Company elected to early adopt this guidance in the current period and has applied it to its evaluation of the license and stock agreement entered into with Selecta (note 14).

Summary of significant accoun25

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2016
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 , 2015 and 2016 as they would be anti-dilutive: December 31, 2014 2015 2016 Convertible preferred shares 10,037,255 — — Unvested restricted common shares 578,994 373,655 251,809 Options issued and outstanding 2,264,497 3,071,372 4,181,993

Marketable securities (Tables)

Marketable securities (Tables)	12 Months Ended
	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Marketable securities	The following table summarizes the available-for-sale securities held at December 31, 2016 : Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2016 U.S. government agency $ 133,690,267 $ 10,907 $ (85,714 ) $ 133,615,460 Corporate securities $ 126,253,903 $ — $ (726,579 ) $ 125,527,324

Business acquisition of Genab27

Business acquisition of Genable (Tables)	12 Months Ended
	Dec. 31, 2016
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block]	The following table summarizes the fair values of the tangible and intangible assets acquired and liabilities assumed at the acquisition date: Cash acquired $ 196,307 Other current assets 102,506 Acquired in-process research and development 15,490,000 Goodwill 1,160,104 Total assets assumed 16,948,917 Other non-current liabilities 254,753 Deferred tax liability 1,000,235 Total liabilities assumed 1,254,988 Total allocation of purchase price $ 15,693,929

Fair value of financial instr28

Fair value of financial instruments (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 293,530,590 — — At December 31, 2016: Assets: Money market funds (included in cash and cash equivalents) $ 53,452,424 — — Corporate securities (included in cash and cash equivalents) $ 5,029,838 Marketable securities - U.S. government agencies $ 133,615,460 — — Marketable securities - corporate securities $ 122,144,692 3,382,632

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	Property and equipment consist of the following: December 31, 2015 2016 Laboratory equipment $ 4,415,121 $ 5,833,616 Computer and software 158,280 530,663 Furniture and fixtures 470,151 470,151 Office equipment 72,308 537,756 Leasehold improvements 11,274,721 16,981,989 Construction in progress 2,511,636 967,945 Property and equipment, gross 18,902,217 25,322,120 Less accumulated depreciation and amortization (1,902,772 ) (5,527,814 ) Property and equipment, net $ 16,999,445 $ 19,794,306

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2016
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following: December 31, 2015 2016 Compensation and benefits $ 4,880,239 $ 9,613,275 Consulting and professional fees 432,346 995,614 Research and development 978,156 2,717,777 Other 238,522 500,254 $ 6,529,263 $ 13,826,920

Stockholders' equity (Tables)

Stockholders' equity (Tables)	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Summary of Restricted Stock Activity	The following table summarizes restricted stock activity for the instruments discussed above: Number of shares Weighted- average grant date fair value 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 Shares canceled (1,213 ) $ 1.15 Shares vested (172,466 ) $ 3.90 Nonvested shares at December 31, 2016 174,876 $ 5.78 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 Shares granted 59,500 $ 44.35 Shares cancelled (1,000 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at December 31, 2016 76,933 $ 49.90

Stock incentive plans (Tables)

Stock incentive plans (Tables)	12 Months Ended
	Dec. 31, 2016
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, 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 Granted 1,722,950 $ 43.16 Exercised (455,138 ) $ 5.69 Forfeited (157,191 ) $ 21.17 Outstanding at December 31, 2016 4,181,993 $ 33.25 Vested at December 31, 2016 1,159,926 $ 24.58 $ 35,495,985 Vested at December 31, 2016 and expected to vest 4,056,533 $ 33.25 $ 83,508,162 (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 that were in the money at December 31, 2016.
Summary of Weighted-Average Assumptions	The weighted average grant date fair value of the options granted in 2014, 2015 and 2016 was $3.25 , $40.51 and $28.56 per share, respectively, using the Black-Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2014 2015 2016 Expected volatility 87.6 % 76.8 % 75.2 % Risk-free interest rate 1.90 % 1.67 % 1.71 % Expected term (in years) 6.02 6.10 5.95 Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Restricted Common Stock Activity	The following table summarizes restricted stock activity for the instruments discussed above: Number of shares Weighted- average grant date fair value 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 Shares canceled (1,213 ) $ 1.15 Shares vested (172,466 ) $ 3.90 Nonvested shares at December 31, 2016 174,876 $ 5.78 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 Shares granted 59,500 $ 44.35 Shares cancelled (1,000 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at December 31, 2016 76,933 $ 49.90

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2017 $ 4,410,126 2018 4,495,760 2019 4,293,484 2020 4,405,160 2021 4,398,678 2022 and thereafter 19,592,238 Total $ 41,595,446

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of income tax benefit computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: Year Ended December 31, 2014 2015 2016 Federal income tax benefit at statutory rate 34.0 % 34.0 % 34.0 % Loss prior to C corporation conversion (7.1 ) — — State and local tax, net of federal benefit 7.3 8.0 10.4 Permanent differences (0.6 ) (2.7 ) 3.0 Tax credits 7.3 13.1 9.1 Change in valuation allowance (40.9 ) (52.4 ) (56.5 ) 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, 2015 2016 Deferred tax assets (liabilities): Net operating loss carryforwards $ 11,385,094 $ 55,710,846 Tax credit carryforwards 11,694,170 28,327,477 Stock based compensation 3,363,805 10,342,547 Deferred rent 3,581,023 3,356,430 Deferred revenue 6,376,558 4,057,998 Accruals and other 2,024,542 9,262,530 Total deferred tax assets 38,425,192 111,057,828 Less valuation allowance (33,826,206 ) (107,616,937 ) Net deferred tax assets $ 4,598,986 $ 3,440,891 Intangible assets $ — $ (1,936,250 ) Fixed assets (4,598,986 ) (2,504,876 ) Total deferred tax liabilities $ (4,598,986 ) $ (4,441,126 ) Net deferred tax liability $ — $ (1,000,235 )

Selected quarterly financial 35

Selected quarterly financial data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly financial information	The following table contains quarterly financial information for 2015 and 2016 . 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. 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 ) 2016 (1) First Quarter Second Quarter Third Quarter Fourth Quarter Total Revenues $ 1,288,628 $ 1,288,629 $ 1,302,789 $ 16,302,789 $ 20,182,835 Research and development 18,251,900 19,621,536 22,384,109 26,121,860 86,379,405 Acquired in-process research and development — — — 11,132,146 11,132,146 General and administrative 8,873,861 10,676,752 12,049,954 16,469,750 48,070,317 Total operating expenses 27,125,761 30,298,288 34,434,063 53,723,756 145,581,868 Loss from operations (25,837,133 ) (29,009,659 ) (33,131,274 ) (37,420,967 ) (125,399,033 ) Net loss $ (25,576,711 ) $ (28,676,115 ) $ (32,562,407 ) $ (36,837,294 ) $ (123,652,527 ) Basic and diluted net loss per common share $ (0.95 ) $ (1.04 ) $ (1.07 ) $ (1.21 ) $ (4.29 ) (1) The sum of the quarterly per share amounts may not equal per share amounts reported for the year due to rounding.

Background - Additional informa

Background - Additional information (Details)	Jun. 20, 2016USD ($)$ / sharesshares	Dec. 28, 2015USD ($)$ / sharesshares	Feb. 04, 2015USD ($)$ / sharesshares	Dec. 31, 2016USD ($)segmentshares	Dec. 31, 2015USD ($)shares	Dec. 31, 2014USD ($)
Class of Stock [Line Items]
Number of operating segments | segment				1
Aggregate gross proceeds from IPO | $			$ 185,200,000
Net proceeds from IPO | $	$ 127,600,000	$ 99,400,000	$ 168,900,000
Proceeds from follow-on public offering | $	$ 136,100,000	$ 106,500,000		$ 126,908,733	$ 270,400,216	$ 0
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO (in shares) | shares	3,025,000	2,266,995		3,025,000	10,316,995
Stock issuance price per share (in usd per share) | $ / shares	$ 45	$ 47
Preferred stock, shares issued upon conversion (in shares) | shares			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 (in shares) | shares			8,050,000
Stock issuance price per share (in usd per share) | $ / shares			$ 23

Development-stage risks - Addit

Development-stage risks - Additional information (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Development-stage Risk [Abstract]
Retained earnings (accumulated deficit)	$ 252,380,911	$ 128,728,384

Summary of significant accoun38

Summary of significant accounting policies - Additional information (Details)	Jan. 16, 2015	Dec. 31, 2013USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Property, Plant and Equipment [Line Items]
Impairment charges of long-lived assets		$ 0	$ 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 accoun39

Summary of significant accounting policies - Schedule of antidilutive securities excluded from computation of diluted weighted average shares outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	0	0	10,037,255
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	251,809	373,655	578,994
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	4,181,993	3,071,372	2,264,497

Marketable securities - Schedul

Marketable securities - Schedule of available-for-sale securities (Details)	12 Months Ended
	Dec. 31, 2016USD ($)
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Remaining maturity period	2 years
U.S. Government Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 133,690,267
Unrealized gains	10,907
Unrealized losses	(85,714)
Fair value	133,615,460
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	126,253,903
Unrealized gains	0
Unrealized losses	(726,579)
Fair value	$ 125,527,324

Business acquisition of Genab41

Business acquisition of Genable - Additional information (Details) - USD ($)	Mar. 07, 2016	Dec. 31, 2016
Genable [Member]
Business Acquisition [Line Items]
Payments to acquire business	$ 6,100,000
Fair value of common stock	9,200,000
Purchase price	15,300,000
Acquisition related costs	$ 300,000
Common Stock [Member] | Genable [Member]
Business Acquisition [Line Items]
Business acquisition, equity issued (in shares)	265,000
Genable [Member] | Genable [Member]
Business Acquisition [Line Items]
Receivable due	$ 500,000
Revenue Commissioners, Ireland [Member] | Foreign Tax Authority [Member]
Business Acquisition [Line Items]
Operating loss carryforwards		$ 936,015
In process research and development | Genable [Member]
Business Acquisition [Line Items]
Acquired in-process research and development	$ 15,500,000

Fair value of financial instr42

Fair value of financial instruments - Summary of assets and liabilities measured at fair value on recurring basis (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 53,452,424	$ 293,530,590
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	$ 0
Fair Value Measurements Recurring [Member] | Corporate Securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	5,029,838
U.S. Government Agency [Member]
Assets:
Fair value	133,615,460
U.S. Government Agency [Member] | Fair Value Measurements Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Fair value	133,615,460
U.S. Government Agency [Member] | Fair Value Measurements Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Fair value	0
U.S. Government Agency [Member] | Fair Value Measurements Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Fair value	0
Corporate Securities [Member]
Assets:
Fair value	125,527,324
Corporate Securities [Member] | Fair Value Measurements Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Fair value	122,144,692
Corporate Securities [Member] | Fair Value Measurements Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Fair value	$ 3,382,632

Business acquisition of Genab43

Business acquisition of Genable - Recognition and measurement of assets acquired and liabilities assumed (Details) - USD ($)	Dec. 31, 2016	Mar. 07, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,160,104		$ 0
Genable [Member]
Business Acquisition [Line Items]
Cash acquired		$ 196,307
Other current assets		102,506
Acquired in-process research and development		15,490,000
Goodwill		1,160,104
Total assets assumed		16,948,917
Other non-current liabilities		254,753
Deferred tax liability		1,000,235
Total liabilities assumed		1,254,988
Total allocation of purchase price		$ 15,693,929

Property and equipment, net - S

Property and equipment, net - Schedule of property and equipment, net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 25,322,120	$ 18,902,217
Less accumulated depreciation and amortization	(5,527,814)	(1,902,772)
Property and equipment, net	19,794,306	16,999,445
Depreciation and amortization expense	3,634,349	1,732,983	$ 169,790
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	5,833,616	4,415,121
Computer and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	530,663	158,280
Furniture & Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	470,151	470,151
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	537,756	72,308
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	16,981,989	11,274,721
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 967,945	$ 2,511,636

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of accrued expenses (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Compensation and benefits	$ 9,613,275	$ 4,880,239
Consulting and professional fees	995,614	432,346
Research and development	2,717,777	978,156
Other	500,254	238,522
Accrued expenses	$ 13,826,920	$ 6,529,263

Debt - Additional Information (

Debt - Additional Information (Details) - Commonwealth of Pennsylvania [Member] - Machinery and Equipment and Other Assets Purchased (MELF Loan) [Member]	1 Months Ended
	Aug. 31, 2016USD ($)
Line of Credit Facility [Line Items]
Loan amount	$ 1,600,000
Debt term	5 years
Principle and interest payment	$ 28,928
Stated percentage rate	3.25%
Debt interest expense	$ 14,533

Stockholders' Equity - Addition

Stockholders' Equity - Additional information (Details) - USD ($)	Feb. 04, 2015	Dec. 31, 2014	May 31, 2014	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Common stock, shares authorized				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
Accumulated dividends	$ 1,300,000
Grant date fair value of restricted stock issued in connection with license agreement														$ 750,000
Research and development				$ 26,121,860	$ 22,384,109	$ 19,621,536	$ 18,251,900	$ 16,554,779	$ 11,796,455	$ 9,343,972	$ 8,334,108	$ 86,379,405	$ 46,029,314	$ 16,351,005
Document Fiscal Year Focus												2,016
Preferred stock, shares issued				0				0				0	0
Preferred stock, $0.001 par value. Authorized, 5,000,000 shares; no shares issued or outstanding				$ 0				$ 0				$ 0	$ 0
Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)	10,200,050
Preferred shares converted into number of common stock (in shares)	10,200,050
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 A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
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 (in shares)			0.2
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
Minimum percentage of approval by outstanding preferred share holders for preferred stock conversion to common stock			87.50%
Preferred stock dividend rate			8.00%
Preferred stock, shares issued			45,186,334
Preferred stock, $0.001 par value. Authorized, 5,000,000 shares; no shares issued or outstanding			$ 72,400,000
Series B Convertible Preferred Stock [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)			0.2
Series 1 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)			0.2
Series 2 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)			0.2
Series 3 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)			0.03883773
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				$ 600,000								$ 600,000
Unrecognized compensation expense related to restricted common shares expected to be recognized, weighted-average period												7 months 18 days
Restricted Shares [Member] | General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense												$ 100,000	100,000	1,200,000
Restricted Shares [Member] | Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense												$ 1,300,000	$ 1,800,000	1,000,000
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 (Details) - Restricted Shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Number of shares (in shares):
Nonvested beginning balance (in shares)	348,555	578,994
Shares vested (in shares)	(172,466)	(230,439)
Shares canceled (in shares)	(1,213)
Nonvested ending balance (in shares)	174,876	348,555
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in usd per share)	$ 4.83	$ 4.46
Shares vested (in usd per share)	3.90	3.87
Shares canceled (in dollars per share)	1.15
Nonvested shares ending balance (in usd per share)	$ 5.78	$ 4.83

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Dec. 31, 2016	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 4 months
Weighted average remaining contractual term of options exercisable			7 years 6 months 10 days
Weighted average grant date fair value of options (in usd per share)			$ 28.56	$ 40.51	$ 3.25
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to the performance-based restricted common stock			$ 0
Grants in period (in shares)			8,012
ESPP [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 0.1
ESPP [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			0.1
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock options			$ 64.3
Weighted average period recognition for unrecognized compensation expense			2 years 10 months 18 days
Intrinsic value of options exercised			$ 21.1	$ 10.4
Employee Stock Option [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			9.2	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			$ 13.2	$ 5.9	$ 0.5
Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period recognition for unrecognized compensation expense			1 year 9 months 50 days
Nonvested shares (in shares)			76,933	25,100	0
Unrecognized compensation expense related to the performance-based restricted common stock			$ 3.3
Performance-Based Restricted Common Stock [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			0.2	$ 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			$ 0.4	$ 0.6
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants (in shares)		209,500	430,542
Common stock reserved for issuance (in shares)		1,830,000
Maximum shares authorized (in shares)		2,543,299
Shares of common stock (in shares)		1,724,000
Percentage of common stock outstanding		4.00%
Shares for ESPP automatically increased (in shares)	1,083,313
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]
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
2015 Employee Stock Purchase Plan [Member] | ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)		220,000	482,816
Shares of common stock (in shares)		440,000
Percentage of common stock outstanding		1.00%
Shares for ESPP automatically increased (in shares)	270,828
Certain Employees and Consultants [Member] | Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares (in shares)			43,600

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of stock option activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Number of shares
Outstanding beginning balance (in shares)	3,071,372	2,264,497
Granted (in shares)	1,722,950	1,031,700
Exercised (in shares)	(455,138)	(221,825)
Forfeited (in shares)	(157,191)	(3,000)
Outstanding ending balance (in shares)	4,181,993	3,071,372
Vested, outstanding (in shares)	1,159,926
Vested and expected to vest, outstanding (in shares)	4,056,533
Weighted-average exercise price
Outstanding beginning balance (in usd per share)	$ 22.99	$ 4.52
Granted (in usd per share)	43.16	59.80
Exercised (in usd per share)	5.69	5.03
Forfeited (in usd per share)	21.17	75.57
Outstanding ending balance (in usd per share)	33.25	$ 22.99
Vested, outstanding (in usd per share)	24.58
Vested and expected to vest, outstanding (in usd per share)	$ 33.25
Vested, outstanding, aggregate intrinsic value	$ 35,495,985
Vested and expected to vest, outstanding, aggregate intrinsic value	$ 83,508,162

Stock Incentive Plans - Summa51

Stock Incentive Plans - Summary of weighted-average assumptions (Details) - Employee Stock Option [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	75.20%	76.80%	87.60%
Risk-free interest rate	1.71%	1.67%	1.90%
Expected term	5 years 11 months 12 days	6 years 1 month 6 days	6 years 7 days
Expected dividend yield	0.00%	0.00%	0.00%

Stock Incentive Plans - Summa52

Stock Incentive Plans - Summary of restricted common stock activity (Details) - Performance-Based Restricted Common Stock [Member] - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Number of shares (in shares):
Nonvested beginning balance (in shares)	25,100	0
Shares granted (in shares)	59,500	49,750
Shares canceled (in shares)	(1,000)
Shares vested (in shares)	(6,667)	(24,650)
Nonvested ending balance (in shares)	76,933	25,100
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in usd per share)	$ 60.03	$ 0
Shares granted (in usd per share)	44.35	60.03
Shares canceled (in dollars per share)	60.03
Shares vested (in usd per share)	37.06	60.03
Nonvested shares ending balance (in usd per share)	$ 49.90	$ 60.03

Commitments and contingencies -

Commitments and contingencies - Additional information (Details)	1 Months Ended		12 Months Ended
	Nov. 30, 2016USD ($)ft²	Dec. 31, 2014USD ($)shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Feb. 29, 2016ft²	Nov. 30, 2015ft²
Property Subject to or Available for Operating Lease [Line Items]
Long-term deferred rent			$ 7,498,419	$ 8,084,509
Current portion of deferred rent			771,196	715,959
Operating leases, rent expense			1,900,000	$ 1,000,000
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,017			4,410,126
2,018			4,495,760
2,019			4,293,484
2,020			4,405,160
2,021			4,398,678
2022 and thereafter			19,592,238
Total			41,595,446
University of Pennsylvania [Member] | License Agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
Issuance of restricted stock in connection with license agreement (in shares) | shares		200,000
Lease of Laboratory and Office Space [Member]
Property Subject to or Available for Operating Lease [Line Items]
Allowance for leasehold improvements					$ 8,000,000
Long-term deferred rent			7,500,000
Current portion of deferred rent			$ 800,000
Lease of Office Space [Member] | Corporate HQ [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of real estate property (in sq ft) | ft²	49,000					6,500
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 (in sq ft) | ft²							5,400
Sublease of Office Space [Member] | Corporate HQ [Member]
Property Subject to or Available for Operating Lease [Line Items]
Allowance for leasehold improvements	$ 3,200,000
Area of real estate property (in sq ft) | 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

Related-party transactions - Ad

Related-party transactions - Additional information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
General and administrative related to agreements	$ 16,469,750	$ 12,049,954	$ 10,676,752	$ 8,873,861	$ 6,872,493	$ 6,461,675	$ 6,333,123	$ 3,684,880	$ 48,070,317	$ 23,352,171	$ 7,863,256
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	1,100,000				200,000				1,100,000	200,000
Accounts payable due to related party	$ 900,000				$ 1,700,000				900,000	1,700,000
CHOP [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements									7,400,000	5,200,000	6,000,000
General and administrative related to agreements											49,393
CHOP [Member] | General and Administrative Expense [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs									$ 900,000	$ 700,000	$ 600,000

Collaboration and license agr55

Collaboration and license agreements - Additional information (Details) - USD ($)	Apr. 30, 2015	Dec. 31, 2016	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	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		$ 5,168,674			$ 5,168,674	$ 5,182,835
Long term deferred revenue		3,865,885			3,865,885	9,034,559
Purchase of acquired in-process research and development					11,132,146	0	$ 0
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	20,200,000	600,000
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
Current portion of deferred revenue		5,200,000			$ 5,200,000
Long term deferred revenue		3,900,000			3,900,000
Revenue recognition from aggregate milestone payments					$ 230,000,000
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					$ 110,000,000
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
Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current		$ 10,000,000
Shares purchased (in shares)		197,238			197,238
Investment shares purchased, amount					$ 5,000,000
Payments to acquire project		$ 15,000,000
Purchase of acquired in-process research and development		$ 11,100,000
Development and Regulatory Milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration agreement expiration, period		90 days
Milestone payments		$ 65,000,000
Milestone payment, option period		3 years
Commercial Milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments		$ 365,000,000
Research and Development Expense [Member] | Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursement revenue					$ 2,400,000	$ 1,300,000	$ 100,000
Minimum [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fees		1,400,000
Maximum [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fees		2,000,000
Milestone payments		$ 430,000,000
Milestone payment, rights to fund percentage		50.00%
Scenario, Forecast [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current				$ 5,000,000
Investment shares purchased, amount				$ 10,000,000
Common Stock [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project		$ 3,900,000
Licensing Agreements [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project		$ 11,100,000

Income taxes - Effective rate r

Income taxes - Effective rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal income tax benefit at statutory rate	34.00%	34.00%	34.00%
Loss prior to C corporation conversion	(0.00%)	(0.00%)	(7.10%)
State and local tax, net of federal benefit	10.40%	8.00%	7.30%
Permanent differences	3.00%	(2.70%)	(0.60%)
Tax credits	9.10%	13.10%	7.30%
Change in valuation allowance	(56.50%)	(52.40%)	(40.90%)
Effective income tax rate	0.00%	0.00%	0.00%

Income taxes - Deferred tax ass

Income taxes - Deferred tax assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	$ 55,710,846	$ 11,385,094
Tax credit carryforwards	28,327,477	11,694,170
Stock based compensation	10,342,547	3,363,805
Deferred rent	3,356,430	3,581,023
Deferred revenue	4,057,998	6,376,558
Accruals and other	9,262,530	2,024,542
Total deferred tax assets	111,057,828	38,425,192
Less valuation allowance	(107,616,937)	(33,826,206)
Net deferred tax assets	3,440,891	4,598,986
Intangible assets	(1,936,250)	0
Fixed assets	(2,504,876)	(4,598,986)
Total deferred tax liabilities	(4,441,126)	(4,598,986)
Net deferred tax liability	(1,000,235)	0
Valuation allowance changes	73,800,000	$ 23,400,000
Domestic Tax Authority [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards	121,900,000
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	1,600,000
Domestic Tax Authority [Member] | Orphan Drug Tax Credit Carryforward [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	26,700,000
State and Local Jurisdiction [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards	$ 119,700,000

Selected quarterly financial 58

Selected quarterly financial data (unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 16,302,789	$ 1,302,789	$ 1,288,629	$ 1,288,628	$ 17,197,789	$ 1,302,789	$ 1,288,629	$ 2,274,467	$ 20,182,835	$ 22,063,674	$ 633,932
Research and development	26,121,860	22,384,109	19,621,536	18,251,900	16,554,779	11,796,455	9,343,972	8,334,108	86,379,405	46,029,314	16,351,005
Acquired in-process research and development	11,132,146	0	0	0					11,132,146	0	750,000
General and administrative	16,469,750	12,049,954	10,676,752	8,873,861	6,872,493	6,461,675	6,333,123	3,684,880	48,070,317	23,352,171	7,863,256
Total operating expenses	(53,723,756)	(34,434,063)	(30,298,288)	(27,125,761)	(23,427,272)	(18,258,130)	(15,677,095)	(12,018,988)	(145,581,868)	(69,381,485)	(24,964,261)
Loss from operations	(37,420,967)	(33,131,274)	(29,009,659)	(25,837,133)	(6,229,483)	(16,955,341)	(14,388,466)	(9,744,521)	(125,399,033)	(47,317,811)	(24,330,329)
Net loss	$ (36,837,294)	$ (32,562,407)	$ (28,676,115)	$ (25,576,711)	$ (6,154,869)	$ (16,900,560)	$ (14,336,842)	$ (9,733,507)	$ (123,652,527)	$ (47,125,778)	$ (24,324,809)
Basic and diluted net loss per common share (in usd per share)	$ (1.21)	$ (1.07)	$ (1.04)	$ (0.95)	$ (0.25)	$ (0.70)	$ (0.60)	$ (0.58)	$ (4.29)	$ (2.10)	$ (4.64)