Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 15, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	Blueprint Medicines Corp
Entity Central Index Key	0001597264
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 1,953,767,657
Entity Common Stock, Shares Outstanding		43,704,031
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 400,304	$ 52,069
Investments, available-for-sale	273,052	162,090
Unbilled accounts receivable		3,577
Prepaid expenses and other current assets	12,149	2,689
Total current assets	685,505	220,425
Investments, available-for-sale		54,059
Property and equipment, net	24,363	6,188
Restricted cash	4,555	1,267
Other assets	1,314	856
Total assets	715,737	282,795
Current liabilities:
Accounts payable	3,744	2,211
Accrued expenses	30,541	11,746
Current portion of deferred revenue	5,373	11,426
Current portion of lease incentive obligation	1,714	578
Current portion of term loan payable	1,518	2,551
Total current liabilities	42,890	28,512
Deferred rent, net of current portion	4,129	932
Deferred revenue, net of current portion	30,000	35,809
Lease incentive obligation, net of current portion	14,617	2,792
Term loan payable, net of current portion		1,518
Other long term liabilities	131	154
Commitments (Note 10)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.001 par value; 120,000,000 shares authorized; 43,577,526 and 33,125,479 shares issued at December 31, 2017 and December 31, 2016, respectively; 43,577,526 and 33,123,354 shares outstanding at December 31, 2017 and December 31, 2016, respectively	43	33
Additional paid-in capital	979,785	420,533
Accumulated other comprehensive loss	(269)	(18)
Accumulated deficit	(355,589)	(207,470)
Total stockholders' equity	623,970	213,078
Total liabilities and stockholders' equity	$ 715,737	$ 282,795

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred Stock Disclosures
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common Stock Disclosures
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	43,577,526	33,125,479
Common Stock, shares outstanding (in shares)	43,577,526	33,123,354

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Collaboration revenue	$ 21,426	$ 27,772	$ 11,400
Operating expenses:
Research and development	144,687	81,131	48,588
General and administrative	27,986	19,218	14,456
Total operating expenses	172,673	100,349	63,044
Other income (expense):
Other income (expense), net	3,349	551	(429)
Interest expense	(221)	(469)	(696)
Total other income (expense)	3,128	82	(1,125)
Net loss	(148,119)	(72,495)	(52,769)
Other comprehensive loss:
Unrealized loss on investments	(251)	(18)
Comprehensive loss	(148,370)	(72,513)	(52,769)
Reconciliation of net loss applicable to common stockholders:
Net loss	(148,119)	(72,495)	(52,769)
Convertible preferred stock dividends			(3,153)
Net loss applicable to common stockholders	$ (148,119)	$ (72,495)	$ (55,922)
Net loss per share applicable to common stockholders - basic and diluted (in dollars per share)	$ (3.92)	$ (2.64)	$ (3.07)
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted	37,793	27,492	18,236

Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Series A convertible preferred stock	Series B convertible preferred stock	Series C convertible preferred stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	Total
Beginning Balance at Dec. 31, 2014	$ 39,958	$ 24,985	$ 49,868
Beginning Balance (in shares at Dec. 31, 2014	40,000,000	20,916,663	24,154,589
Increase (Decrease) in Convertible Preferred Stock
Conversion of preferred stock into common stock	$ (39,958)	$ (24,985)	$ (49,868)
Conversion of preferred stock into common stock (in shares)	(40,000,000)	(20,916,663)	(24,154,589)
Beginning Balance at Dec. 31, 2014				$ 2	$ 2,822		$ (82,206)	$ (79,382)
Beginning Balance (in shares) at Dec. 31, 2014				1,626,738
Increase (Decrease) in Stockholders' Equity
Conversion of preferred stock into common stock				$ 15	114,792			114,807
Conversion of preferred stock into common stock (in shares)				15,467,479
Initial public offering / Follow on offering, net of issuance costs				$ 9	154,743			154,752
Initial public offering / Follow on offering, net of issuance costs (in shares)				9,367,708
Reclassification of warrant					810			810
Issuance of common stock upon warrant exercise (in shares)				32,438
Issuance of common stock under stock plan				$ 1	580			581
Issuance of common stock under stock plan (in shares)				571,195
Stock-based compensation expense					5,180			5,180
Net loss							(52,769)	(52,769)
Ending Balance at Dec. 31, 2015				$ 27	278,927		(134,975)	143,979
Ending Balance (in shares) at Dec. 31, 2015				27,065,558
Increase (Decrease) in Stockholders' Equity
Initial public offering / Follow on offering, net of issuance costs				$ 6	134,543			134,549
Initial public offering / Follow on offering, net of issuance costs (in shares)				5,750,000
Issuance of common stock under stock plan					546			546
Issuance of common stock under stock plan (in shares)				284,471
Purchase of common stock under ESPP					377			377
Purchase of common stock under ESPP (in shares)				23,325
Stock-based compensation expense					6,140			6,140
Unrealized loss on available-for-sale securities, net of tax						$ (18)		(18)
Net loss							(72,495)	(72,495)
Ending Balance at Dec. 31, 2016				$ 33	420,533	(18)	(207,470)	$ 213,078
Ending Balance (in shares) at Dec. 31, 2016				33,123,354				33,125,479
Increase (Decrease) in Stockholders' Equity
Initial public offering / Follow on offering, net of issuance costs				$ 10	541,366			$ 541,376
Initial public offering / Follow on offering, net of issuance costs (in shares)				10,009,259
Issuance of common stock under stock plan					4,887			4,887
Issuance of common stock under stock plan (in shares)				428,210
Purchase of common stock under ESPP					476			476
Purchase of common stock under ESPP (in shares)				16,703
Stock-based compensation expense					12,523			12,523
Unrealized loss on available-for-sale securities, net of tax						(251)		(251)
Net loss							(148,119)	(148,119)
Ending Balance at Dec. 31, 2017				$ 43	$ 979,785	$ (269)	$ (355,589)	$ 623,970
Ending Balance (in shares) at Dec. 31, 2017				43,577,526				43,577,526

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (148,119)	$ (72,495)	$ (52,769)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,578	1,582	948
Noncash interest expense	32	73	109
Change in fair value of warrant liability			445
Stock-based compensation	12,523	6,140	5,180
Accretion of premiums and discounts on investments	(320)	301
Changes in assets and liabilities:
Unbilled accounts receivable	3,577	(163)	(3,414)
Prepaid expenses and other current assets	(7,266)	1,470	(1,482)
Other assets	(458)	(304)	(552)
Accounts payable	1,532	4	1,391
Accrued expenses	14,597	5,773	1,956
Deferred revenue	(11,861)	33,594	13,641
Deferred rent	14,320	(488)	2,871
Net cash used in operating activities	(119,865)	(24,513)	(31,676)
Investing activities
Purchases of property and equipment	(15,512)	(2,354)	(4,883)
Restricted cash	(3,501)	119	(1,196)
Purchases of investments	(360,835)	(264,467)
Maturities of investments	304,000	48,000
Net cash used in investing activities	(75,848)	(218,702)	(6,079)
Financing activities
Principal payments on loan payable	(2,583)	(3,333)	(1,806)
Payment of offering costs	(965)	(143)	(2,046)
Proceeds from public offering of common stock, net of commissions and underwriting discounts	542,225	135,125	156,815
Proceeds from issuance of common stock, net of repurchases	5,271	928	259
Net cash provided by financing activities	543,948	132,577	153,222
Net increase (decrease) in cash and cash equivalents	348,235	(110,638)	115,467
Cash and cash equivalents at beginning of period	52,069	162,707	47,240
Cash and cash equivalents at end of period	400,304	52,069	162,707
Supplemental cash flow information
Public offering costs incurred but unpaid at period end	317	433
Property and equipment purchases unpaid at period end	3,947		1,244
Conversion of convertible preferred stock into common stock			114,808
Reclassification of warrant liability to additional paid-in-capital			810
Cash paid for interest	$ 151	$ 316	$ 452

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2017
Disclosure Text Block
Nature of Business	1. Nature of Business Blueprint Medicines Corporation (the Company), a Delaware corporation incorporated on October 14, 2008, is a biopharmaceutical company focused on developing potentially transformational medicines to improve the lives of patients with genomically defined cancers and rare diseases. The Company’s approach is to leverage its novel target discovery engine to systematically and reproducibly identify kinases that are drivers of diseases in genomically defined patient populations and to craft highly selective and potent drug candidates that may provide significant and durable clinical responses for patients without adequate treatment options. The Company is devoting substantially all of its efforts to research and development, initial market development and raising capital. The Company is subject to a number of risks similar to those of other early stage companies, including dependence on key individuals; establishing safety and efficacy in clinical trials for its drug candidates; the need to develop commercially viable drug candidates; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its drug candidates. If the Company is unable to raise capital when needed or on attractive terms, it would be forced to delay, reduce, eliminate or out‑license certain of its research and development programs or future commercialization efforts. On May 5, 2015, the Company completed an initial public offering (IPO) of its common stock, which resulted in the sale of 9,367,708 shares of its common stock at a price to the public of $18.00 per share, including 1,221,874 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of $154.8 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company. On December 13, 2016, the Company closed its underwritten public offering of 5,750,000 shares of its common stock at a price to the public of $25.00 per share, including 750,000 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of $134.5 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company. On April 4, 2017, the Company closed its underwritten public offering of 5,750,000 shares of its common stock at a price to the public of $40.00 per share, including 750,000 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of approximately $215.6 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company. On December 15, 2017, the Company closed its underwritten public offering of 4,259,259 shares of its common stock at a price to the public of $81.00 per share, including 555,555 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of approximately $325.7 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company. As of December 31, 2017, the Company had cash, cash equivalents and investments of $673.4 million. Based on the Company’s current plans, the Company expects that its existing cash, cash equivalents and investments, excluding any potential option fees and milestone payments under its existing collaboration with F. Hoffmann-La Roche Ltd and Hoffmann-La Roche Inc. (collectively, Roche), will be sufficient to enable it to fund its operating expenses and capital expenditure requirements into the middle of 2020.

Summary of Significant Accounting Policies and Recent Accounting Pronouncements

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Summary of Significant Accounting Policies and Recent Accounting Pronouncements

2. Summary of Significant Accounting Policies and Recent Accounting Pronouncements

Basis of Presentation

The audited consolidated financial statements of the Company included herein have been prepared in accordance with accounting principles generally accepted in the United States (GAAP) as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB) and the rules and regulations of the SEC.

The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, Blueprint Medicines Security Corporation, which is a Massachusetts subsidiary created to buy, sell and hold securities. All intercompany transactions and balances have been eliminated.

Effective April 10, 2015, the Company effected a 1‑for‑5.5 reverse stock split of the Company’s common stock. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid‑in capital. Due to the underwritten public offerings completed on December 13, 2016, April 4, 2017, and December 15, 2017, there were significant increases in shares outstanding in the years ended December 31, 2017 and 2016, which impacts the year-over-year comparability of the Company’s net loss per share calculations.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires the Company’s management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and in developing the estimates and assumptions that are used in the preparation of the financial statements. Management must apply significant judgment in this process. Management’s estimation process often may yield a range of potentially reasonable estimates and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: stock‑based compensation expense, including estimating the fair value of the Company’s common stock prior to the IPO; revenue recognition; the valuation of liability‑classified warrants prior to the IPO; accrued expenses; and income taxes.

Significant Accounting Policies

The Company’s critical accounting policies are those policies that require the most significant judgments and estimates in the preparation of our financial statements. Management has determined that the Company’s most critical accounting policies are those relating to revenue recognition, accrued research and development expenses, available-for-sale investments and stock-based compensation.

Available-for-Sale Investments

The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current. Available-for-sale securities are maintained by an investment manager and may consist of U.S. Treasury securities and U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, will mark the investment to market through a charge to the Company’s statement of operations and comprehensive loss.

Revenue Recognition

The Company recognizes revenue from license and collaboration agreements in accordance with FASB ASC Topic 605, Revenue Recognition (ASC 605). Accordingly, revenue is recognized when all of the following criteria are met:

persuasive evidence of an arrangement exists;

delivery has occurred or services have been rendered;

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

collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recognized as deferred revenue in the Company’s balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

When evaluating multiple element arrangements, the Company considers whether the deliverables under the arrangement represent separate units of accounting. This evaluation requires subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have standalone value, based on the consideration of the relevant facts and circumstances for each arrangement. The consideration received is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria are applied to each of the separate units. Deliverables are considered separate units of accounting provided that: (i) the delivered item(s) has value to the customer on a stand-alone basis and (ii) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially in the control of the Company. In assessing whether an item has stand-alone value, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use the deliverable(s) for their intended purpose without the receipt of the remaining element(s), whether the value of the deliverable is dependent on the undelivered item(s) and whether there are other vendors that can provide the undelivered element(s). The Company’s collaboration agreements with Alexion and Roche do not contain a general right of return relative to the delivered item(s).

Arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method. Then, the applicable revenue recognition criteria in ASC 605-25 are applied to each of the separate units of accounting in determining the appropriate period and pattern of recognition. The Company determines the selling price of a unit of accounting following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, the Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (VSOE) of selling price, if available, third-party evidence (TPE) of selling price if VSOE is not available, or best estimate of selling price (BESP) if neither VSOE nor TPE is available. The Company typically uses BESP to estimate the selling price, since it generally does not have VSOE or TPE of selling price for its units of accounting. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

In the event that an element of a multiple element arrangement does not represent a separate unit of accounting, the Company recognizes revenue from the combined element over the period over which it expects to fulfill its performance obligations or as undelivered items are delivered, as appropriate, if all of the other revenue recognition criteria in ASC 605-25 are met. If the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. If there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to complete its performance obligations. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight-line method or proportional performance method, as applicable, as of the period ending date.

The Company’s multiple-element revenue arrangements may include the following:

Exclusive Licenses

The deliverables under the Company’s collaboration agreements may include exclusive licenses to research, develop, manufacture and commercialize licensed products. To account for this element of an arrangement, management evaluates whether an exclusive license has stand-alone value from the undelivered elements based on the consideration of the relevant facts and circumstances of the arrangement, including the research and development capabilities of the collaboration partner. The Company may recognize the arrangement consideration allocated to licenses upon delivery of the license if facts and circumstances indicate that the license has stand-alone value from the undelivered elements, which generally include research and development services. The Company defers arrangement consideration allocated to licenses if facts and circumstances indicate that the delivered license does not have stand-alone value from the undelivered elements.

When management believes a license does not have stand-alone value from the other deliverables to be provided in the arrangement, the Company recognizes revenue attributed to the license on a proportional basis over the Company’s contractual or estimated performance period, which is typically the term of the Company’s research and development obligations. If management cannot reasonably estimate when the Company’s performance obligation ends, then revenue is deferred until management can reasonably estimate when the performance obligation ends. The periods over which revenue should be recognized are subject to estimates by management and may change over the course of the research and development and licensing agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods.

Research and Development Services

The deliverables under the Company’s collaboration agreements may include research and development services to be performed by the Company on behalf of the partner. Payments or reimbursements resulting from the Company’s research and development efforts are recognized as the services are performed and presented on a gross basis because the Company is the principal for such efforts, so long as there is persuasive evidence of an arrangement, the fee is fixed or determinable, and collection of the related amount is reasonably assured.

Milestone Revenue

The Company’s collaboration agreements may include contingent milestone payments related to specified pre-clinical milestones, development milestones and sales-based commercial milestones.

At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether:

the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone;

the consideration relates solely to past performance; and

the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement.

The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone and the level of effort and investment required to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. Milestones that are not considered substantive are accounted for as license payments and recognized over the remaining period of performance from the date of achievement of the milestone. Milestones that are considered substantive will be recognized in their entirety upon successful accomplishment of the milestone with a cumulative catch up adjustments, assuming all other revenue recognition criteria are met.

Royalty Revenue.

The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met.

Comprehensive Income (Loss)

Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non‑owner sources. Other comprehensive income (loss) consisted of unrealized gains and losses on investments for the year ended December 31, 2017 and 2016. For the year ended December 31, 2015, comprehensive loss was equal to net loss.

Research and Development Costs

Expenditures relating to research and development are expensed in the period incurred. Research and development expenses consist of both internal and external costs associated with the development of the Company’s selective cancer therapies and building of its discovery platform.

In certain circumstances, the Company is required to make nonrefundable advance payments to vendors for goods or services that will be received in the future for use in research and development activities. In such circumstances, the nonrefundable advance payments are deferred and capitalized, even when there is no alternative future use for the research and development, until related goods or services are provided. In circumstances where amounts have been paid in excess of costs incurred, the Company records a prepaid expense.

Property and Equipment, Net

Property and equipment consists of lab equipment, furniture and fixtures, computer equipment, software, and leasehold improvements, all of which is stated at cost. Expenditures for maintenance and repairs are recorded to expense as incurred, whereas major betterments are capitalized as additions to property and equipment. Depreciation is recognized over the estimated useful lives of the assets using the straight‑line method.

Impairment of Long‑Lived Assets

The Company continually evaluates whether events or circumstances have occurred that indicate that the estimated remaining useful life of its long‑lived assets may warrant revision or that the carrying value of these assets may be impaired. The Company has not recognized any impairment charges through December 31, 2017.

Warrants

The Company accounts for warrant instruments that either conditionally or unconditionally obligate the issuer to transfer assets and liabilities regardless of the timing of the redemption feature or price, even though the underlying shares may be classified as permanent or temporary equity. These warrants are subject to revaluation at each balance sheet date, and any changes in fair value are recorded as a component of other income (expense), until the earlier of their exercise or expiration or the time the warrants no longer conditionally or unconditionally obligate the Company to transfer assets or liabilities, which occurred upon the IPO, at which time the warrant liability was reclassified to stockholders’ equity.

Stock‑Based Compensation Expense

The Company expenses the fair value of employee stock awards on a straight‑line basis over the requisite service period, which generally is the vesting period. Prior to 2017, expense was recognized net of estimated forfeitures and adjusted to reflect actual forfeitures. Effective January 1, 2017, upon adoption of ASU No. 2016-09, Compensation – Stock Compensation under the modified retrospective approach, the Company recognizes gross stock compensation expense with actual forfeitures recognized as they occur. Compensation cost for restricted stock awards issued to employees is measured using the grant date intrinsic value of the award adjusted to reflect actual forfeitures. The Company estimates the fair value of the options granted to employees at the date of grant using the Black‑Scholes option‑pricing model that requires management to apply judgment and make estimates, including:

expected volatility, which is calculated based on reported volatility data for a representative group of publicly traded companies for which historical information is available. Prior to April 30, 2015, the Company was a privately-held company and lacked company-specific historical and implied volatility information. As such, the Company has used an average of expected volatility based on the volatilities of a representative group of publicly traded biopharmaceutical companies for a period equal to the expected term of the option grant. Beginning in the fourth quarter of 2015, the Company began to include its own volatility into the average calculation. The Company intends to consistently apply this process using representative companies until a sufficient amount of historical information regarding the volatility of its own share price becomes available;

risk‑free interest rate, which is based on the U.S. Treasury yield curve in effect at the time of grant commensurate with the expected term assumption;

expected term, which the Company calculates using the simplified method, as prescribed by the Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share‑Based Payment, as the Company has insufficient historical information regarding stock options to provide a basis for an estimate;

prior to becoming a public company, fair value estimates of the underlying common shares, which were determined using the option‑pricing method (OPM) or a hybrid of the probability‑weighted expected return method and the OPM and were approved by the Company’s board of directors. Upon becoming a public company, the fair value of the underlying common shares equals the closing price of the Company’s stock on The Nasdaq Global Select Market on the date of grant; and

dividend yield which is zero based on the fact that the Company never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

Stock‑based awards issued to non‑employees, including directors for non‑board‑related services, are accounted for based on the fair value of such services received or of the intrinsic value of equity instruments issued, whichever is more reliably measured. These stock‑based awards are revalued at each vesting date and period‑end. Stock‑based awards subject to service‑based vesting conditions are expensed on a straight‑line basis over the vesting period.

Income Taxes

The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the Company’s financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts and the tax bases of the assets and liabilities using the enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance against deferred tax assets is recorded if, based on the weight of the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions using a more‑likely‑than‑not threshold for recognizing and resolving uncertain tax positions. The evaluation of uncertain tax positions is based on factors including, but not limited to, changes in the law, the measurement of tax positions taken or expected to be taken in tax returns, the effective settlement of matters subject to audit, new audit activity, and changes in facts or circumstances related to a tax position.

Concentrations of Credit Risk and Off‑Balance‑Sheet Risk

The Company has no significant off‑balance‑sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk primarily consist of cash and cash equivalents, investments and accounts receivable.

The Company maintains its cash, cash equivalents and investments in a custodian account at high quality financial institutions, and consequently, the Company believes that such funds are subject to minimal credit risk.

Accounts receivable represents amounts due from the Company’s collaboration partner. The Company monitors economic conditions to identify facts or circumstances that may indicate that its accounts receivable is at risk of collection.

Segment and Geographic Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief operating decision maker view the Company’s operations and manage its business as one operating segment. The Company operates only in the United States.

New Accounting Pronouncements- Recently Adopted

In March 2016, the FASB issued ASU No. 2016-09, Compensation – Stock Compensation (ASU No. 2016-09), which amends ASC Topic 718, Compensation – Stock Compensation. The new standard identifies areas for simplification involving several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, an option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, as well as certain classifications on the statement of cash flows. The new standard was effective for the Company on January 1, 2017. The Company applied ASU 2016-09 using a modified retrospective approach and adopted the option to recognize gross stock compensation expense with actual forfeitures recognized as they occur. Given that the application of the estimated forfeiture rate prior to January 1, 2017 resulted in an insignificant reduction in stock-based compensation expense, the cumulative-effect adjustment to retained earnings recognized as of January 1, 2017 was not material to the consolidated financial statements. The adoption of ASU 2016-09 also requires all income tax adjustments to be recognized in the consolidated statements of operations. As the increase in net deferred tax assets is fully offset by a corresponding increase to the deferred tax asset valuation allowance, there was no material impact of the adoption of this standard. The amount of deferred tax assets that had not been previously recognized due to the recognition of excess tax benefits is $1.1 million.

New Accounting Pronouncement

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (ASU 2014-09), which supersedes the revenue recognition requirements in ASC 605-25, Multiple-Element Arrangements and most industry-specific guidance. In addition, the FASB recently issued ASUs 2016-10 and 2016-12, which provide clarifying amendments to ASU 2014-09. ASU 2014-09 and its related amendments will be effective for the Company for interim and annual periods beginning after December 15, 2017. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The update also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. This new guidance will be effective for annual reporting periods (including interim reporting periods within those years) beginning January 1, 2018. Companies have the option of applying this new guidance retrospectively to each prior reporting period presented (the full retrospective method) or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application (the modified retrospective method). The Company will adopt the new standard effective January 1, 2018 using the modified retrospective method.

The Company is assessing, but has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements. Currently, the Company anticipates a potential impact on the revenue recognition method used to recognize revenue under its agreement with Roche as well as the recognition of milestone revenue prior to achievement. The expected impact is further described below. Estimated impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard.

Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. ASC 606 also impacts certain other areas, such as the accounting for costs to obtain or fulfill a contract. The standard also requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers.

With respect to its collaboration with Roche, the Company currently expects that the pattern of revenue recognition under step (v) above will differ from the pattern of revenue recognition under ASC 605. Any change in the timing or pattern of revenue recognition will have a corresponding change to the Company’s deferred revenue balance.

The Company expects the accounting for contingent milestone payments under its collaboration agreements to change under ASC 606. ASC 606 does not contain guidance specific to milestone payments, thereby requiring contingent milestone payments to be considered in accordance with the overall model of ASC 606. Revenue from contingent milestone payments may be recognized earlier under ASC 606 than under ASC 605, based on an assessment of the probability of achievement of the milestone event and the likelihood of a significant reversal of such milestone revenue at each reporting date. This assessment may result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved.

ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts.

In connection with the adoption of these standards, the Company is implementing several new internal controls, including controls to monitor the probability of achievement of contingent milestone payments and the pattern of performance of the performance obligation.

In February 2016, the FASB issued ASU No. 2016-02, Leases (ASU No. 2016-02), which will change the way the Company recognizes its leased assets. ASU No. 2016-02 will require organizations that lease assets—referred to as “lessees”—to recognize on the balance sheet the assets and liabilities representing the rights and obligations created by those leases. ASU No. 2016-02 will also require disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. The standard is effective for annual reporting periods (including interim reporting periods within those years) beginning after December 15, 2018. Early adoption is permitted. The Company is currently evaluating the methods of adoption allowed by the new standard and the effect that adoption of the standard is expected to have on the Company’s consolidated financial statements and related disclosures.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230) (ASU No. 2016-15), which simplifies certain elements of cash flow classification. The new guidance is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU No. 2016-15 is effective for annual periods beginning after December 15, 2017. The Company does not believe the adoption of ASU No. 2016-15 will have a significant impact on the consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (ASU No. 2016-18). The amendments in ASU No. 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows. ASU No. 2016-18 is effective for fiscal years (including interim reporting periods within those years) beginning after December 15, 2017. Early adoption is permitted. A reporting entity must apply the amendments in ASU No. 2016-18 using a full retrospective approach. The Company believes that the adoption of this guidance will not have a significant impact on its consolidated financial statements and related disclosures.

In 2016, the FASB issued ASU No. 2016-01 Financial Instruments (ASU No. 2016-01) related to the recording of financial assets and financial liabilities. Under the amended guidance, equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) are to be measured at fair value with changes in fair value recognized in net income (loss). However, an entity has the option to either measure equity investments without readily determinable fair values either (i) at fair value or (ii) at cost adjusted for changes in observable prices minus impairment. Changes in measurement under either alternative will be recognized in net income (loss). The amended guidance became effective January 1, 2018. As the Company does not currently hold equity securities, ASU No. 2016-01 will not have an impact on the consolidated financial statements at the adoption date. The Company may hold equity securities in the future, at which time the Company will apply the provisions of ASU No. 2016-01 and record changes in the fair value of the equity securities in net income (loss).

Cash Equivalents and Investments

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Cash Equivalents and Investments

3. Cash Equivalents and Investments

Cash equivalents are highly liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. Investments consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

Cash equivalents and investments, available-for-sale, consisted of the following at December 31, 2017 and December 31, 2016 (in thousands):


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	400,304	$	—	$	—	$	400,304
Investments, available-for-sale:
U.S. treasury obligations		348 Days		273,321		—		(269)		273,052
Total			$	673,625	$	—	$	(269)	$	673,356


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2016	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	52,069	$	—	$	—	$	52,069
Investments, available-for-sale:
U.S. treasury obligations		298 Days		216,167		14		(32)		216,149
Total			$	268,236	$	14	$	(32)	$	268,218

Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the years ended December 31, 2017 and 2016, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.

At December 31, 2017 and 2016, the Company held 37 and 33 securities, respectively, that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2017 and 2016 were $273.1 million and $147.1 million, respectively, and there were no securities held by the Company in an unrealized loss position for more than twelve months. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than temporary impairment as of December 31, 2017 and December 31, 2016.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

The fair value hierarchy prioritizes the inputs to valuation techniques used to measure fair value into three broad levels as follows:

Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities.

Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

Financial instruments measured at fair value as of December 31, 2017, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	400,304	$	400,304	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		273,052		273,052		—		—
Total	$	673,356	$	673,356	$	—	$	—

Financial instruments measured at fair value as of December 31, 2016, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2016		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	52,069	$	52,069	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		216,149		216,149		—		—
Total	$	268,218	$	268,218	$	—	$	—

At December 31, 2017 and December 31, 2016, the fair value of the Company’s term loan payable is determined using current applicable rates for similar instruments as of the balance sheet date. The carrying value of the Company’s term loan payable approximates fair value because the Company’s interest rate yield approximates current market rates. The Company’s term loan payable is a Level 3 liability within the fair value hierarchy.

The fair value of the preferred stock warrant liability was determined based on Level 3 inputs and utilizing the Black-Scholes option pricing model (see Note 10). On May 5, 2015, upon completion of the IPO, the warrants to purchase preferred stock converted into warrants to purchase common stock and the Company reclassified the fair value of the warrants as of May 5, 2015 to additional paid-in capital. The following table presents activity in the preferred stock warrant liability during the year ended December 31, 2015 (in thousands):


	Year Ended
	December 31,
	2015
Beginning balance	$	365
Issuance of warrant at fair value		—
Change in fair value		445
Reclassification of fair value to additional paid-in capital		(810)
Ending balance	$	—

- Definition

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

+ References + Details

Restricted Cash	12 Months Ended
Restricted Cash	Dec. 31, 2017
Disclosure Text Block
Restricted Cash	5. Restricted Cash At December 31, 2017 and 2016, $4.8 million and $1.3 million, respectively, of the Company’s cash is restricted by a bank. As of December 31, 2017 and 2016, $4.6 million and $1.3 million, respectively, of the restricted cash was included in long-term assets on the Company’s balance sheet related to security deposits for the lease agreements for the Company’s current and former corporate headquarters. On April 28, 2017, the Company entered into a lease agreement with UP 45/75 Sidney Street, LLC (Landlord) for approximately 99,833 rentable square feet of office and laboratory space located at 45 Sidney Street in Cambridge, Massachusetts. The initial term of the lease agreement commenced on October 1, 2017 and will expire on November 30, 2029. The lease agreement requires the Company to maintain a security deposit with the Landlord in the amount of $3.5 million, which is included in the long-term restricted cash balance on the Company’s balance sheet as of December 31, 2017. The security deposit is subject to reduction by up to $1.0 million during the term of the lease agreement, subject to the satisfaction of specified terms and conditions.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Property and Equipment, Net

6. Property and Equipment, Net

Property and equipment and related accumulated depreciation are as follows (in thousands):


	Estimated
	Useful Life	December 31,			December 31,
	(Years)	2017		2016
Lab equipment	5	$	3,604	$	3,059
Furniture and fixtures	4		837		784
Computer equipment	3		624		768
Leasehold improvements	Term of lease		4,681		4,673
Software	3		263		172
Construction-in-progress			18,998		—
			29,007		9,456
Less: accumulated depreciation and amortization			(4,644)		(3,268)
Total property and equipment, net		$	24,363	$	6,188

Depreciation expense for the years ended December 31, 2017, 2016 and 2015 was $1.6 million, $1.6 million and $0.9 million, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Accrued Expenses

7. Accrued Expenses

Accrued expenses consist of the following (in thousands):


	December 31,		December 31,
	2017		2016
External research and development	$	17,880	$	5,696
Employee compensation		5,765		4,118
Professional fees and other		2,467		1,697
Interest		197		212
Property and equipment costs		4,176		23
Severance		56		—
	$	30,541	$	11,746

Collaborations	12 Months Ended
Collaborations	Dec. 31, 2017
Disclosure Text Block
Collaborations	8. Collaborations Roche In March 2016, the Company entered into a collaboration and license agreement (as amended, Roche agreement) with Roche for the discovery, development and commercialization of up to five small molecule therapeutics targeting kinases believed to be important in cancer immunotherapy, as single products or possibly in combination with other therapeutics. The parties are currently conducting activities for up to five programs under the collaboration, including up to two collaboration programs leveraging the Company’s novel target discovery engine and proprietary compound library to select potential targets. Under the Roche agreement, Roche is granted up to five option rights to obtain an exclusive license to exploit products derived from the collaboration programs in the field of cancer immunotherapy. Such option rights are triggered upon the achievement of Phase 1 proof-of-concept. For up to three of the five collaboration programs, if Roche exercises its option, Roche will receive worldwide, exclusive commercialization rights for the licensed products. For up to two of the five collaboration programs, if Roche exercises its option, the Company will retain commercialization rights in the United States for the licensed products, and Roche will receive commercialization rights outside of the United States for the licensed products. The Company will also retain worldwide rights to any products for which Roche elects not to exercise its applicable option. Prior to Roche’s exercise of an option, the Company will have the lead responsibility for drug discovery and pre-clinical development of all collaboration programs. In addition, the Company will have the lead responsibility for the conduct of all Phase 1 clinical trials other than those Phase 1 clinical trials for any product in combination with Roche’s portfolio of therapeutics, for which Roche will have the right to lead the conduct of such Phase 1 clinical trials. Pursuant to the Roche agreement, the parties will share the costs of Phase 1 development for each collaboration program. In addition, Roche will be responsible for post-Phase 1 development costs for each licensed product for which it retains global commercialization rights, and the Company and Roche will share post-Phase 1 development costs for each licensed product for which the Company retains commercialization rights in the United States. Subject to the terms of the Roche agreement, the Company received an upfront cash payment of $45.0 million and will be eligible to receive up to approximately $965.0 million in contingent option fees and milestone payments related to specified research, pre-clinical, clinical, regulatory and sales-based milestones. Of the total contingent payments, up to approximately $215.0 million are for option fees and milestone payments for research, pre-clinical and clinical development events prior to licensing across all five potential collaboration programs, including contingent milestone payments for initiation of each of the collaboration programs for which the parties will work together to select targets (pre-option exercise milestones). In addition, for any licensed product for which Roche retains worldwide commercialization rights, the Company will be eligible to receive tiered royalties ranging from low double-digits to high-teens on future net sales of the licensed product. For any licensed product for which the Company retains commercialization rights in the United States, the Company and Roche will be eligible to receive tiered royalties ranging from mid-single-digits to low double-digits on future net sales in the other party’s respective territories in which it commercializes the licensed product. The upfront cash payment and any payments for milestones, option fees and royalties are non-refundable, non-creditable and not subject to set-off. The Roche agreement will continue until the date when no royalty or other payment obligations are or will become due, unless earlier terminated in accordance with the terms of the Roche agreement. Prior to its exercise of its first option, Roche may terminate the Roche agreement at will, in whole or on a collaboration target-by-collaboration target basis, upon 120 days’ prior written notice to the Company. Following its exercise of an option, Roche may terminate the Roche agreement at will, in whole, on a collaboration target-by-collaboration target basis, on a collaboration program-by-collaboration program basis or, if a licensed product has been commercially sold, on a country-by-country basis, (i) upon 120 days’ prior written notice if a licensed product has not been commercially sold or (ii) upon 180 days’ prior written notice if a licensed product has been commercially sold. Either party may terminate the Roche agreement for the other party’s uncured material breach or insolvency and in certain other circumstances agreed to by the parties. In certain termination circumstances, the Company is entitled to retain specified licenses to be able to continue to exploit the licensed products. The Company determined that there were five deliverables under the Roche collaboration: (i) a non-transferable, sub-licensable and non-exclusive license to use the Company’s intellectual property and collaboration compounds to conduct research activities;(ii) conducting research and development activities through Phase 1 clinical trials under the research plan; (iii) providing pre-clinical and clinical supply of collaboration compounds; (iv) participation on a joint research committee (JRC) and joint development committee (JDC); and (v) regulatory responsibilities under Phase 1 clinical trials. The Company determined that the license did not have value to Roche on a stand-alone basis due to the specialized nature of the research activities to be provided by the Company that are not available in the marketplace and the fact that the license is to perform research and development only. Therefore, the license has limited value without the performance of the research and development activities and is not separable. The pre-clinical and clinical supply activities are integral to the performance of the research and development activities and can only be used for the performance of such activities, and the regulatory responsibilities are dependent on the research and development activities. The Company determined that the best estimate for the selling price of the JRC and JDC participation was inconsequential. Accordingly, the Company combined the license, pre-clinical and clinical supply, JRC and JDC participation and regulatory responsibilities deliverables with the research and development activities, the last item to be delivered in the arrangement, as one unit of accounting. The Company is recognizing the total allocable arrangement consideration consisting of the upfront payment of $45.0 million as revenue on a straight-line basis over the Company’s best estimate of the period it expects to perform research and development activities. The Company expects the services to be delivered ratably. The Company evaluated whether the option fees that may be received in connection with the Roche agreement are substantive. The Company concluded that the option fees were substantive due to the uncertainty around whether the goals of the collaboration will be achieved, and therefore the options are not a deliverable in the current arrangement. If Roche elects to exercise the options, the exercises and related contingent deliverables would be accounted for as a separate arrangement. The Company evaluated whether the milestones that may be received in connection with the Roche agreement are substantive milestones. Pre-option exercise milestones, of up to $215.0 million, that are expected to be achieved as a result of the Company’s efforts during the performance of the research and development activities are considered substantive and are recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. The development event milestones are not considered substantive because the Company does not contribute effort to the achievement of such milestones as they are expected to be achieved after the performance of the research and development activities. Consideration received with respect to these milestones will be added to the total arrangement consideration that has been allocated to the identified units of accounting. As a result, that amount is recognized as revenue ratably over the period starting from the effective date of the agreement to the date that the Company will complete all of its obligations, with a cumulative catch-up from the effective date through the date of achievement of the milestone. If the consideration is received after the completion of all of the Company’s obligations, the amount will be recognized as revenue immediately. During the years ended December 31, 2017 and 2016, the Company recognized revenue under the Roche collaboration of $5.2 million and $4.5 million, respectively, which represents a portion of the $45.0 million upfront payment. Alexion In March 2015, the Company entered into a research, development and commercialization agreement (Alexion agreement) with Alexion to research, develop and commercialize one or more drug candidates targeting the ALK2 kinase for the treatment of fibrodysplasia ossificans progressiva (FOP). On July 26, 2017, the Company received written notice from Alexion of its election to terminate the Alexion agreement for convenience, and the termination became effective on October 24, 2017. Under the terms of the Alexion agreement, the Company and Alexion agreed to collaborate to research, develop and commercialize one or more drug candidates targeting the ALK2 kinase for the treatment of FOP, which is a rare genetic disease caused by mutations in the ALK2 gene, ACVR1. Effective upon the termination, the Company’s exclusivity obligations under the Alexion collaboration terminated, including without limitation, the Company’s exclusivity obligations with respect to (i) the treatment of FOP, heterotopic ossification or diffuse intrinsic pontine glioma, (ii) ALK2, including both wild-type and mutated forms, and (iii) certain molecules related to the foregoing. In addition, the research term and all licenses granted to Alexion terminated, and certain licenses granted by Alexion to the Company survived and became perpetual, irrevocable and non-terminable. Pursuant to the Alexion agreement, the Company was responsible for research and pre-clinical development activities related to any drug candidates, and Alexion was responsible for all clinical development, manufacturing and commercialization activities related to any drug candidates. In addition, Alexion was responsible for funding 100% of the Company’s research and development costs incurred under the research plan, including pass‑through costs and a negotiated yearly rate per full‑time equivalent for its employees’ time and their associated overhead expenses. As a result of the termination of the Alexion agreement, the Company will not be entitled to receive payment from Alexion for any research and development expenses incurred after the effective date of termination. Prior to termination, the Company had received an aggregate amount of $18.8 million in upfront and milestone payments. The Company received a $15.0 million non‑refundable upfront payment in March 2015 upon execution of the Alexion agreement and was eligible to receive over $250.0 million in payments upon the successful achievement of pre‑specified pre-clinical, clinical, regulatory and commercial milestones. Prior to termination, the Company had received an aggregate amount of $3.8 million in pre-clinical milestone payments. As a result of the termination of the Alexion agreement, the Company will not be entitled to receive payment from Alexion for any additional milestones. Prior to termination, Alexion was required to pay the Company tiered royalties, ranging from mid‑single to low‑double digit percentages, on a country‑by‑country and licensed-product‑by‑licensed product basis, on worldwide net product sales of licensed products. There were no refund provisions in the Alexion agreement. The Company determined that there were three deliverables under the former Alexion collaboration: (i) an exclusive license to research, develop, manufacture and commercialize the licensed products and the compounds in the field in the territory, (ii) conducting research and development activities under the research plan and (iii) participation on a joint steering committee (JSC) and joint project team (JPT). The Company determined that the license did not have value to Alexion on a stand-alone basis due to the specialized nature of the research services to be provided by the Company that are not available in the marketplace. Therefore, the deliverables were not separable and, accordingly, the license, undelivered research and development activities and JSC and JPT participation were a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition model on the final deliverable. Under the Alexion agreement, the last deliverable to be completed is its research and development activities and participation on the JSC and JPT, which were expected to be delivered over the same performance period. The Company recognized the remaining deferred revenue balance related to the upfront payment and non-substantive milestone payment previously received under the former collaboration with Alexion, utilizing the proportional performance model over the remaining period of performance, which ended October 24, 2017. The Company evaluated whether the milestones that it was eligible to receive in connection with the Alexion agreement were substantive or non-substantive milestones. The Company concluded that the first pre-clinical milestone payment under the former Alexion collaboration was non-substantive due to the certainty at the date the arrangement was entered into that the event will be achieved. In the second quarter of 2015, the Company achieved the first pre-clinical milestone under the former Alexion collaboration and received a $1.8 million payment from Alexion. The Company recognized revenues from the related milestone payment over the remaining period of performance. The remaining non-refundable pre-clinical milestones that the Company was eligible to achieve as a result of the Company’s efforts prior to the termination were considered substantive and would have been recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria were met. The Company has recognized and received an aggregate of $2.0 million in substantive milestones through December 31, 2017. Milestones that were expected to be achieved after the period of substantial involvement were not considered substantive because the Company did not contribute effort to the achievement of such milestones. During the year ended December 31, 2017, the Company recognized revenue under the former Alexion collaboration of $16.2 million, which represents $9.5 million of reimbursable research and development costs, as well as a portion of the $15.0 million upfront payment and the $1.8 million non-substantive milestone payment previously received. During the year ended December 31, 2016, the Company recognized revenue under the Alexion agreement of $23.3 million, which represents $14.6 million of reimbursable research and development costs, $1.8 million in milestone payments that were recognized upon achievement, as well as a portion of the $15.0 million upfront payment and the $1.8 million non-substantive milestone payment previously received. During the year ended December 31, 2017, the Company received $12.7 million related to reimbursable research and development costs under the former Alexion collaboration.

Term Loan

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Term Loan

9. Term Loan

In May 2013, the Company entered into a loan and security agreement with Silicon Valley Bank (the 2013 Term Loan), which provided for up to $5.0 million in funding, to be made available in three tranches. Loan advances accrue interest at a fixed rate of 2% above the prime rate. In November 2014, the Company amended the 2013 Term Loan to allow the Company to borrow an additional $5.0 million (the 2014 Term Loan). The Company accounted for the amendment as a modification to the existing 2013 Term Loan. The Company immediately drew the additional $5.0 million under the 2014 Term Loan and was required to make interest‑only payments until December 1, 2015, and consecutive monthly payments of principal, plus accrued interest, over the remaining term through November 2018. The Company is required to pay a fee of 4% of the total loan advances at the end of the term of each of the 2013 Term Loan and the 2014 Term Loan. The fee is being accreted to interest expense over the term of the 2013 Term Loan and the 2014 Term Loan. In the event of prepayment, the Company is obligated to pay 1% to 2% of the amount of the outstanding principal depending upon the timing of the prepayment. There are no financial covenants associated with the loan and security agreement.

The 2013 Term Loan and 2014 Term Loan are collateralized by a blanket lien on all corporate assets, excluding intellectual property, and by a negative pledge of the Company’s intellectual property. The term loan contains covenants, including restrictions on dividends and default provisions. The 2013 Term Loan and 2014 Term Loan contain customary default provisions that include material adverse events, as defined therein. The Company has determined that the risk of subjective acceleration under the material adverse events clause is remote and therefore has classified the outstanding principal in current liabilities based on scheduled principal payments.

The Company assessed all terms and features of the 2013 Term Loan and the 2014 Term Loan in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the term loan, including put and call features. The Company determined that all features of each of the 2013 Term Loan and the 2014 Term Loan are clearly and closely associated with a debt host and do not require bifurcation as a derivative liability, or the fair value of the feature is immaterial to the Company’s financial statements. The Company will continue to reassess the features on a quarterly basis to determine if they require separate accounting.

Scheduled monthly principal payments on the term loan, as of December 31, 2017, are as follows (in thousands):


2018		1,528
Total	$	1,528

Warrants

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Warrants

10. Warrants

In connection with the 2013 Term Loan, the Company issued a warrant to Silicon Valley Bank to purchase 150,000 shares of Series A convertible preferred stock at an exercise price of $1.00 per share (the Series A Warrant). In connection with the 2014 Term Loan, the Company issued an additional warrant to Silicon Valley Bank to purchase 83,333 shares of Series B convertible preferred stock at an exercise price of $1.20 per share (the Series B Warrant). Both warrants were exercisable immediately and have a ten‑year life.

The Company initially valued the Series A Warrant and the Series B Warrant at issuance and at the balance sheet dates using the Black‑Scholes option pricing model. The significant assumptions used in estimating the fair value of the warrants include the volatility of the stock underlying the warrant, risk‑free interest rate, estimated fair value of the preferred stock underlying the warrant, and the estimated term of the warrant. The fair value of the preferred stock underlying the warrants was estimated using the implied value from the common stock valuations on those dates.

In accordance with ASC 480, the characteristics of these warrants and the rights and privileges of the underlying preferred stock resulted in the classification of these warrants as a liability, and they were re‑measured to the‑then current fair value at each balance sheet date through the completion of the IPO. Re‑measurement gains or losses were recorded in other income (expense) in the statements of operations and comprehensive loss. Changes in the fair value of the warrants represented a recurring measurement that was classified within Level 3 of the fair value hierarchy wherein fair value is estimated using significant unobservable inputs. The Company recorded $0.4 million of expense associated with the change in fair value of the warrants in the year ended December 31, 2015 equal to the change in fair value of the warrants from December 31, 2014 to May 5, 2015.

Upon completion of the IPO, the Series A Warrant became exercisable for 27,272 shares of the common stock at an exercise price of $5.50 per share, and the Series B Warrant became exercisable for 15,151 shares of the common stock at an exercise price of $6.60 per share. On the date of the conversion of the warrants, the Company revalued the outstanding warrants using the Black-Scholes option pricing model with the following assumptions:


	Series A Warrant			Series B Warrant
	May 5,			May 5,
	2015			2015
Fair value of underlying instrument	$	20.82		$	20.82
Expected volatility		91.58	%		87.75	%
Expected term (in years)		8.1			9.5
Risk-free interest rate		2.06	%		2.19	%
Expected dividend yield		—	%		—	%

The fair value of the warrants at May 5, 2015 was $0.8 million and the Company reclassified the balance to additional paid-in capital.

On May 13, 2015, Silicon Valley Bank exercised the Series A Warrant and the Series B Warrant pursuant to the cashless exercise feature of the warrants. In connection with the exercise of the Series A Warrant under the 2013 Term Loan, the Company issued 21,281 shares of common stock to Silicon Valley Bank. Warrants to purchase 5,991 shares of common stock were cancelled as payment for the aggregate exercise price of the Series A Warrant to Silicon Valley Bank. In connection with the exercise of the Series B Warrant under the 2014 Term Loan, the Company issued 11,157 shares of common stock. Warrants to purchase 3,994 shares of common stock were cancelled as payment for the aggregate exercise price of the Series B Warrant.

The Company recorded a debt discount upon issuance of the warrants, which is being accreted as interest expense over the remaining term of the loan. The Company recorded interest expense related to the Series A Warrant and the Series B Warrant of less than $0.1 million in each of the years ended December 31, 2017, 2016 and 2015.

Stock Awards

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Stock Awards

11. Stock Awards

2015 Stock Option and Incentive Plan

In 2015, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan (the 2015 Plan), which replaced the Company’s 2011 Stock Option and Grant Plan, as amended (the 2011 Plan). The 2015 Plan includes incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock, restricted stock units, unrestricted stock, performance share awards and cash‑based awards. The Company initially reserved a total of 1,460,084 shares of common stock for the issuance of awards under the 2015 Plan. The 2015 Plan provides that the number of shares reserved and available for issuance under the 2015 Plan will be cumulatively increased on January 1 of each calendar year by 4% of the number of shares of common stock issued and outstanding on the immediately preceding December 31 or such lesser amount as specified by the compensation committee of the board of directors. For the calendar year beginning January 1, 2017 and 2018, the number of shares reserved for issuance under the 2015 Plan was increased by 1,325,019 and 1,743,101 shares, respectively. In addition, the total number of shares reserved for issuance is subject to adjustment in the event of a stock split, stock dividend or other change in our capitalization. At December 31, 2017, there were 1,769,028 shares available for future grant under the 2015 Plan.

Awards

Options and restricted stock awards granted by the Company generally vest ratably over four years, with a one‑year cliff for new employee awards, and are exercisable from the date of grant for a period of ten years.

A summary of the Company’s unvested restricted stock and related information follows:


		Weighted-Average
		Grant Date
	Shares	Fair Value
Unvested at December 31, 2016	2,125	$	1.25
Vested	(2,125)		1.25
Repurchased	—		—
Unvested at December 31, 2017	—		—

The total fair value of restricted stock that vested during the years ended December 31, 2017, 2016 and 2015 was $0.1 million, $2.7 million and $4.9 million, respectively.

A summary of the Company’s stock option activity and related information follows:


		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(1)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2016	2,622,741	$	11.67	8.30	$	44,025
Granted	1,212,972		42.62
Exercised	(426,085)		11.47
Canceled	(105,462)		30.50
Outstanding at December 31, 2017	3,304,166	$	22.45	8.04	$	174,985
Exercisable at December 31, 2017	1,432,958	$	11.63	7.34	$	91,389

(1)

Intrinsic value represents the amount by which the fair market value as of December 31, 2017 of the underlying common stock exceeds the exercise price of the option.

The fair value of stock options is estimated on the grant date using the Black‑Scholes option‑pricing model based on the following weighted average assumptions:



	Year Ended
	December 31, 2017		December 31, 2016		December 31, 2015
Risk-free interest rate	2.07	%	1.55	%	1.66	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.0		6.0		6.0
Expected stock price volatility	74.58	%	75.94	%	85.43	%

The weighted‑average grant date fair value of options granted in the years ended December 31, 2017, 2016 and 2015 was $28.04, $13.06 and $8.55, respectively. The total intrinsic value of options exercised in the years ended December 31, 2017, 2016 and 2015 was $17.8 million, $3.1 million and $6.8 million, respectively.

Total stock‑based compensation expense recognized for all stock‑based compensation awards in the statements of operations and comprehensive loss is as follows (in thousands):


	Year Ended
	December 31,
	2017		2016		2015
Research and development	$	6,296	$	2,674	$	2,148
General and administrative		6,227		3,466		3,032
Total stock-based compensation expense	$	12,523	$	6,140	$	5,180

At December 31, 2017, there was $35.2 million of total unrecognized compensation cost related to non-vested stock awards, which is expected to be recognized over a weighted‑average period of 2.41 years. Due to an operating loss, the Company does not record tax benefits associated with stock‑based compensation or option exercises. Tax benefit will be recorded when realized.

2015 Employee Stock Purchase Plan

In 2015, the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which became effective upon the closing of the IPO in May 2015. The Company initially reserved a total of 243,347 shares of common stock for issuance under the 2015 ESPP. The 2015 ESPP provides that the number of shares reserved and available for issuance under the 2015 ESPP will be cumulatively increased on January 1 of each calendar years by 1% of the number of shares of common stock issued and outstanding on the immediately preceding December 31 or such lesser amount as specified by the compensation committee of the board of directors. For the calendar year beginning January 1, 2017 and 2018, the number of shares reserved for issuance under the 2015 ESPP was increased by 331,254 and 435,775 shares, respectively. The Company issued 16,703 and 23,325 shares under the ESPP during the years ended December 31, 2017 and 2016, respectively.

Net Loss per Share

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Net Loss per Share

12. Net Loss per Share

Basic net loss per share applicable to common stockholders is calculated by dividing net loss applicable to common stockholders by the weighted average shares outstanding during the period, without consideration for common stock equivalents. Net loss applicable to common stockholders is calculated by adjusting the net loss of the Company for cumulative preferred stock dividends. Diluted net loss per share applicable to common stockholders is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period. For purposes of the dilutive net loss per share applicable to common stockholders calculation, convertible preferred stock, warrants, stock options, and unvested restricted stock are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share applicable to common stockholders, as their effect would be anti‑dilutive; therefore, basic and diluted net loss per share applicable to common stockholders were the same for all periods presented as a result of the Company’s net loss.

The following common stock equivalents were excluded from the calculation of diluted net loss per share applicable to common stockholders for the periods indicated because including them would have had an anti‑dilutive effect:


	Year Ended
	December 31,
	2017	2016	2015
Stock options	3,304,166	2,622,741	1,802,802
Unvested restricted stock	—	2,125	130,495
Total	3,304,166	2,624,866	1,933,297

The weighted average number of common shares used in net loss per share applicable to common stockholders on a basic and diluted basis were 37,792,929, 27,491,669 and 18,235,614 for the years ended December 31, 2017, 2016 and 2015, respectively.

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2017
Disclosure Text Block
Convertible Preferred Stock	13. Convertible Preferred Stock Upon the closing of the IPO in May 2015, Series A Convertible Preferred Stock, Series B Convertible Preferred Stock and Series C Convertible Preferred Stock automatically converted into 15,467,479 shares of common stock. In addition, upon closing of the IPO, the Company’s board of directors was authorized, without action by the stockholders, to designate and issue up to an aggregate of 5,000,000 shares of preferred stock in one or more series. The board of directors can designate the rights, preferences and privileges of the shares of each series and any of its qualifications, limitations or restrictions. As of December 31, 2017, no shares of preferred stock were issued or outstanding.

Income Taxes

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Income Taxes

14. Income Taxes

A reconciliation of the U.S. statutory income tax rate to the Company’s effective tax rate is as follows for the years ended December 31, 2017, 2016 and 2015:


	Year Ended
	December 31,
	2017		2016		2015
Federal income tax (benefit) at statutory rate	34.00	%	34.00	%	34.00	%
Permanent differences	0.49		(2.74)		(3.30)
Federal research and development credits	0.97		1.02		1.09
Federal orphan drug credits	6.40		6.63		0.55
State income tax, net of federal benefit	5.40		4.81		4.72
Other	0.83		(0.78)		0.56
Deferred rate change	(27.58)		—		—
Change in valuation allowance	(20.54)		(42.94)		(37.62)
Effective income tax rate	(0.03)	%	—	%	—	%

The Company had net losses in all periods presented and therefore has not recognized any federal or state income tax expense.

The Company’s deferred tax assets and liabilities consist of the following:


	Year Ended
	December 31,
	2017		2016		2015
Deferred tax assets:
Net operating loss carryforwards	$	78,829	$	69,443	$	48,291
Research and development credit carryforwards		5,950		3,833		2,829
Orphan drug credit carryforwards		14,574		5,095		289
Accrued expenses and other		4,660		2,914		1,374
Deferred revenue		9,664		2,627		—
Deferred lease incentive		4,462		1,324		1,551
Deferred rent		1,142		366		331
Total gross deferred tax asset		119,281		85,602		54,665
Deferred tax liability		(4,801)		(1,535)		(1,702)
Debt discount		(2)		(14)		(39)
Valuation allowance		(114,478)		(84,053)		(52,924)
Net deferred tax asset	$	—	$	—	$	—

Management has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, and has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets, and as a result, a valuation allowance of $114.5 million, $84.1 million and $52.9 million has been established at December 31, 2017, 2016 and 2015, respectively. The change in the valuation allowance was $30.4 million, $31.1 million and $19.8 million for the years ended December 31, 2017, 2016 and 2015, respectively. The Company has incurred net operating losses (NOL) since inception. At December 31, 2017, the Company had federal and state NOL carryforwards of $288.6 million and $288.5 million, respectively, which expire beginning in 2030. As of December 31, 2017, the Company had federal and state research and development tax credit carryforwards of $4.2 million and $2.1 million, respectively, which expire beginning in 2025. As of December 31, 2017, the Company had federal orphan drug credits of $14.6 million, which expire beginning in 2035 and state investment tax credits of $0.1 million, which expire beginning in 2018.

The Internal Revenue Code of 1986, as amended (the Code), provides for a limitation of the annual use of net operating losses and other tax attributes (such as research and development tax credit carryforwards) following certain ownership changes (as defined by the Code) that could limit the Company’s ability to utilize these carryforwards. At this time, the Company has not completed a study to assess whether an ownership change under Section 382 of the Code has occurred, or whether there have been multiple ownership changes since the Company’s formation. The Company may have experienced ownership changes, as defined by the Code, as a result of past financing transactions. Accordingly, the Company’s ability to utilize the aforementioned carryforwards may be limited. In addition, U.S. tax laws limit the time during which these carryforwards may be applied against future taxes. Therefore, the Company may not be able to take full advantage of these carryforwards for federal or state income tax purposes.

Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statements of operations and comprehensive loss. As of December 31, 2017 and 2016, the Company has no accrued interest related to uncertain tax positions. In many cases, the Company’s uncertain tax positions are related to years that remain subject to examination by relevant tax authorities. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state, and local income tax authorities for all tax years in which a loss carryforward is available.

For all years through December 31, 2017, the Company generated research credits but has not completed 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. 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 valuation allowance. Thus, there would be no impact to the balance sheets or statements of operations and comprehensive loss if an adjustment were required.

On December 22, 2017, H.R.1, known as the Tax Cuts and Jobs Act, was enacted. This new law did not have a significant impact on the Company’s consolidated financial statements for the year ended December 31, 2017 because it maintains a valuation allowance on the majority of its net operating losses and other deferred tax assets. However, the reduction of the U.S. federal corporate tax rate from 35% to 21% resulted in increases to the amounts reflected in “Deferred rate change” in the Company’s tax reconciliation table above for the year ended December 31, 2017 compared to the years ended December 31, 2016 and 2015. The change in the U.S. federal corporate tax rate, which is effective January 1, 2018, is also reflected in the Company’s deferred tax table above.

On December 22, 2017, the SEC staff issued SAB 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of H.R.1. The Company has recognized the provisional tax impacts related to the revaluation of deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts, possibly materially, due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of H.R.1. The Company will complete the accounting treatment for H.R.1 by December 22, 2018, which is one year from enactment date of H.R.1.

Commitments

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Commitments

15. Commitments

On February 12, 2015, the Company entered into a lease for approximately 38,500 rentable square feet of office and laboratory space in Cambridge, Massachusetts, which the Company gained control over on June 15, 2015, and occupancy commenced in October 2015. The lease ends on October 31, 2022. The Company has an option to extend the lease for five additional years. The lease has a total commitment of $17.8 million over the seven year term. The Company has agreed to pay an initial annual base rent of approximately $2.3 million, which rises periodically until it reaches approximately $2.8 million. The Company is recording rent expense on a straight-line basis through the end of the lease term. The Company has recorded deferred rent on the consolidated balance sheet at December 31, 2017, accordingly. The lease provides the Company with an allowance for leasehold improvements of $4.3 million. The Company accounts for leasehold improvement incentives as a reduction to rent expense ratably over the lease term. The balance from the leasehold improvement incentives is included in lease incentive obligations on the balance sheet. The lease agreement required the Company to pay a security deposit of $1.3 million, which is recorded in restricted cash on the Company’s balance sheet.

On April 28, 2017, the Company entered into a lease agreement for approximately 99,833 rentable square feet of office and laboratory space located at 45 Sidney Street in Cambridge, Massachusetts. The initial term of the lease agreement commenced on October 1, 2017 and will expire on November 30, 2029. The lease agreement also provides the Company with an option to extend the lease agreement for two consecutive five-year periods at the then fair market annual rent, as defined in the lease agreement, as well as a right of first offer with respect to leasing additional space adjacent to the existing leased premises. During the initial term of the lease agreement, the Company has agreed to pay an initial annual base rent of approximately $7.7 million, which rises periodically until it reaches approximately $10.6 million in the last year of the initial term. The Company is recording rent expense on a straight-line basis through the end of the lease term. The Company has recorded deferred rent on the consolidated balance sheet at December 31, 2017 accordingly. The landlord has also agreed to provide the Company with a tenant improvement allowance of approximately $14.2 million for improvements to be made to the premises. The Company accounts for leasehold improvement incentives as a reduction to rent expense ratably over the lease term. The lease agreements required the Company to pay a security deposit of $3.5 million, which is recorded in restricted cash on the Company’s balance sheet.

The future minimum lease payments at December 31, 2017, are as follows (in thousands):


2018		10,174
2019		10,479
2020		10,794
2021		11,118
2022		10,977
Thereafter		67,541
Total minimum lease payments	$	121,083

The Company records rent expense under its lease agreements on a straight line basis. For the years ended December 31, 2017, 2016, and 2015, rent expense was $6.3 million, $1.8 million, and $1.8 million, respectively.

Defined Contribution Benefit Plan	12 Months Ended
Defined Contribution Benefit Plan	Dec. 31, 2017
Disclosure Text Block
Defined Contribution Benefit Plan	16. Defined Contribution Benefit Plan The Company maintains a 401(k) plan for employees (the 401(k) Plan). The 401(k) Plan is intended to qualify under Section 401(k) of the Internal Revenue Service Code of 1986, as amended, so that contributions to the 401(k) Plan by employees or by the Company, and the investment earnings thereon, are not taxable to the employees until withdrawn from the 401(k) Plan, and so that contributions by the Company, if any, will be deductible by the Company when made. Under the 401(k) Plan, employees may elect to reduce their current compensation by up to the statutorily prescribed annual limit and to have the amount of such reduction contributed to the 401(k) Plan. The 401(k) Plan permits the Company to make contributions up to the limits allowed by law on behalf of all eligible employees. Effective September 1, 2015, the Company instituted an employer match of 50% of eligible contributions up to 6% of employee contributions. For the years ended December 31, 2017, 2016 and 2015, the Company contributed $0.5 million, $0.4 million and $0.1 million, respectively, to the 401(k) Plan.

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2017

Disclosure Text Block

Selected Quarterly Financial Data (unaudited)

17. Selected Quarterly Financial Data (unaudited)

The following table contains selected quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.


		Three Months Ended
		March 31, 2017		June 30, 2017		September 30, 2017		December 31, 2017
		(in thousands, except per share data)
Total revenue	$	5,840	$	5,890	$	8,068	$	1,628
Total operating expenses		34,170		40,104		46,678		51,721
Total other income (expense), net		353		802		907		1,066
Net loss	$	(27,977)	$	(33,412)	$	(37,703)	$	(49,027)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.84)	$	(0.86)	$	(0.96)	$	(1.23)


		Three Months Ended
		March 31, 2016		June 30, 2016		September 30, 2016		December 31, 2016
		(in thousands, except per share data)
Total revenue		6,856	$	7,065	$	6,160	$	7,691
Total operating expenses	$	22,281		25,961		23,043		29,064
Total other expense, net		(79)		2		49		110
Net loss	$	(15,504)	$	(18,894)	$	(16,834)	$	(21,263)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.57)	$	(0.70)	$	(0.62)	$	(0.75)

Subsequent Event	12 Months Ended
Subsequent Event	Dec. 31, 2017
Disclosure Text Block
Subsequent Event	18. Subsequent Events In the first quarter of 2018, the Company subleased its former corporate headquarters at 38 Sidney Street, Cambridge, Massachusetts through October 31, 2020. Subject to the terms of the sublease agreement and the master lease agreement, including a right of recapture by the Company, the sublessee has the option to extend the sublease through October 31, 2022. The sublease includes a total commitment of $8.2 million over the 32 month term of the sublease agreement.

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)

12 Months Ended

Dec. 31, 2017

Policy Text Blocks

Basis of Presentation

Use of Estimates

Significant Accounting Policies

Available-for-sale Investments

Available-for-Sale Investments

Revenue recognition

Revenue Recognition

persuasive evidence of an arrangement exists;

delivery has occurred or services have been rendered;

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

collectability is reasonably assured.

The Company’s multiple-element revenue arrangements may include the following:

Exclusive Licenses

Research and Development Services

Milestone Revenue

The Company’s collaboration agreements may include contingent milestone payments related to specified pre-clinical milestones, development milestones and sales-based commercial milestones.

the consideration relates solely to past performance; and

the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement.

Royalty Revenue.

Comprehensive Income (Loss)

Research and Development Costs

Property and Equipment, Net

Impairment of Long Lived Assets

Impairment of Long‑Lived Assets

Warrants

Stock-Based Compensation Expense

Stock‑Based Compensation Expense

risk‑free interest rate, which is based on the U.S. Treasury yield curve in effect at the time of grant commensurate with the expected term assumption;

dividend yield which is zero based on the fact that the Company never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

Income Taxes

Concentrations of Credit Risk and Off-Balance-Sheet Risk

Concentrations of Credit Risk and Off‑Balance‑Sheet Risk

Segment and Geographic Information

New Accounting Pronouncements- Recently Adopted

New Accounting Pronouncement

Cash Equivalents and Investments (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of cash equivalents and investments, available-for-sale

Cash equivalents and investments, available-for-sale, consisted of the following at December 31, 2017 and December 31, 2016 (in thousands):


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	400,304	$	—	$	—	$	400,304
Investments, available-for-sale:
U.S. treasury obligations		348 Days		273,321		—		(269)		273,052
Total			$	673,625	$	—	$	(269)	$	673,356


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2016	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	52,069	$	—	$	—	$	52,069
Investments, available-for-sale:
U.S. treasury obligations		298 Days		216,167		14		(32)		216,149
Total			$	268,236	$	14	$	(32)	$	268,218

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of financial instruments measured at fair value

Financial instruments measured at fair value as of December 31, 2017, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	400,304	$	400,304	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		273,052		273,052		—		—
Total	$	673,356	$	673,356	$	—	$	—

Financial instruments measured at fair value as of December 31, 2016, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2016		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	52,069	$	52,069	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		216,149		216,149		—		—
Total	$	268,218	$	268,218	$	—	$	—

Summary of changes in the fair value of the warrants which represents a recurring measurement that is classified within Level 3 of the fair value hierarchy wherein fair value is estimated


	Year Ended
	December 31,
	2015
Beginning balance	$	365
Issuance of warrant at fair value		—
Change in fair value		445
Reclassification of fair value to additional paid-in capital		(810)
Ending balance	$	—

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of property and equipment

Property and equipment and related accumulated depreciation are as follows (in thousands):


	Estimated
	Useful Life	December 31,			December 31,
	(Years)	2017		2016
Lab equipment	5	$	3,604	$	3,059
Furniture and fixtures	4		837		784
Computer equipment	3		624		768
Leasehold improvements	Term of lease		4,681		4,673
Software	3		263		172
Construction-in-progress			18,998		—
			29,007		9,456
Less: accumulated depreciation and amortization			(4,644)		(3,268)
Total property and equipment, net		$	24,363	$	6,188

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of accrued expenses

Accrued expenses consist of the following (in thousands):


	December 31,		December 31,
	2017		2016
External research and development	$	17,880	$	5,696
Employee compensation		5,765		4,118
Professional fees and other		2,467		1,697
Interest		197		212
Property and equipment costs		4,176		23
Severance		56		—
	$	30,541	$	11,746

Term Loan (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of principal payments on the term loan

Scheduled monthly principal payments on the term loan, as of December 31, 2017, are as follows (in thousands):


2018		1,528
Total	$	1,528

Warrants (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of weighted average assumptions of warrants in Black Scholes option pricing model


	Series A Warrant			Series B Warrant
	May 5,			May 5,
	2015			2015
Fair value of underlying instrument	$	20.82		$	20.82
Expected volatility		91.58	%		87.75	%
Expected term (in years)		8.1			9.5
Risk-free interest rate		2.06	%		2.19	%
Expected dividend yield		—	%		—	%

Stock Awards (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Summary of unvested restricted stock


		Weighted-Average
		Grant Date
	Shares	Fair Value
Unvested at December 31, 2016	2,125	$	1.25
Vested	(2,125)		1.25
Repurchased	—		—
Unvested at December 31, 2017	—		—

Summary of stock option activity


		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(1)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2016	2,622,741	$	11.67	8.30	$	44,025
Granted	1,212,972		42.62
Exercised	(426,085)		11.47
Canceled	(105,462)		30.50
Outstanding at December 31, 2017	3,304,166	$	22.45	8.04	$	174,985
Exercisable at December 31, 2017	1,432,958	$	11.63	7.34	$	91,389

(1)

Intrinsic value represents the amount by which the fair market value as of December 31, 2017 of the underlying common stock exceeds the exercise price of the option.

Assumptions used in estimating fair value of stock options on grant date



	Year Ended
	December 31, 2017		December 31, 2016		December 31, 2015
Risk-free interest rate	2.07	%	1.55	%	1.66	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.0		6.0		6.0
Expected stock price volatility	74.58	%	75.94	%	85.43	%

Summary of stock-based compensation expense recognized in statements of operations

Total stock‑based compensation expense recognized for all stock‑based compensation awards in the statements of operations and comprehensive loss is as follows (in thousands):


	Year Ended
	December 31,
	2017		2016		2015
Research and development	$	6,296	$	2,674	$	2,148
General and administrative		6,227		3,466		3,032
Total stock-based compensation expense	$	12,523	$	6,140	$	5,180

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of common stock equivalents excluded from calculation of diluted net loss per share applicable to common stockholders


	Year Ended
	December 31,
	2017	2016	2015
Stock options	3,304,166	2,622,741	1,802,802
Unvested restricted stock	—	2,125	130,495
Total	3,304,166	2,624,866	1,933,297

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of the reconciliation of the effective tax rate


	Year Ended
	December 31,
	2017		2016		2015
Federal income tax (benefit) at statutory rate	34.00	%	34.00	%	34.00	%
Permanent differences	0.49		(2.74)		(3.30)
Federal research and development credits	0.97		1.02		1.09
Federal orphan drug credits	6.40		6.63		0.55
State income tax, net of federal benefit	5.40		4.81		4.72
Other	0.83		(0.78)		0.56
Deferred rate change	(27.58)		—		—
Change in valuation allowance	(20.54)		(42.94)		(37.62)
Effective income tax rate	(0.03)	%	—	%	—	%

Schedule of the deferred tax assets and liabilities


	Year Ended
	December 31,
	2017		2016		2015
Deferred tax assets:
Net operating loss carryforwards	$	78,829	$	69,443	$	48,291
Research and development credit carryforwards		5,950		3,833		2,829
Orphan drug credit carryforwards		14,574		5,095		289
Accrued expenses and other		4,660		2,914		1,374
Deferred revenue		9,664		2,627		—
Deferred lease incentive		4,462		1,324		1,551
Deferred rent		1,142		366		331
Total gross deferred tax asset		119,281		85,602		54,665
Deferred tax liability		(4,801)		(1,535)		(1,702)
Debt discount		(2)		(14)		(39)
Valuation allowance		(114,478)		(84,053)		(52,924)
Net deferred tax asset	$	—	$	—	$	—

Commitments (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of future minimum lease payments

The future minimum lease payments at December 31, 2017, are as follows (in thousands):


2018		10,174
2019		10,479
2020		10,794
2021		11,118
2022		10,977
Thereafter		67,541
Total minimum lease payments	$	121,083

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Table Text Blocks

Schedule of quarterly financial information


		Three Months Ended
		March 31, 2017		June 30, 2017		September 30, 2017		December 31, 2017
		(in thousands, except per share data)
Total revenue	$	5,840	$	5,890	$	8,068	$	1,628
Total operating expenses		34,170		40,104		46,678		51,721
Total other income (expense), net		353		802		907		1,066
Net loss	$	(27,977)	$	(33,412)	$	(37,703)	$	(49,027)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.84)	$	(0.86)	$	(0.96)	$	(1.23)


		Three Months Ended
		March 31, 2016		June 30, 2016		September 30, 2016		December 31, 2016
		(in thousands, except per share data)
Total revenue		6,856	$	7,065	$	6,160	$	7,691
Total operating expenses	$	22,281		25,961		23,043		29,064
Total other expense, net		(79)		2		49		110
Net loss	$	(15,504)	$	(18,894)	$	(16,834)	$	(21,263)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.57)	$	(0.70)	$	(0.62)	$	(0.75)

Nature of Business (Details) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Dec. 15, 2017	Apr. 04, 2017	Dec. 13, 2016	May 05, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nature of Business
Proceeds from issuance of IPO shares					$ 542,225	$ 135,125	$ 156,815
Proceeds from issuance of common stock					5,271	$ 928	$ 259
Cash, cash equivalents and investments					$ 673,400
Common Stock
Nature of Business
Stock sold (in shares)					10,009,259	5,750,000	9,367,708
IPO \| Common Stock
Nature of Business
Stock sold (in shares)				9,367,708
Share price (in dollars per share)				$ 18.00
Proceeds from issuance of IPO shares				$ 154,800
Follow-on Offering
Nature of Business
Proceeds from issuance of common stock			$ 134,500
Follow-on Offering \| Common Stock
Nature of Business
Stock sold (in shares)	4,259,259	5,750,000	5,750,000
Share price (in dollars per share)	$ 81.00	$ 40.00	$ 25.00
Proceeds from issuance of common stock	$ 325,700	$ 215,600
Underwriters' Option \| Common Stock
Nature of Business
Stock sold (in shares)	555,555	750,000	750,000	1,221,874

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Impairment of Long-Lived Assets (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Impairment of Long Lived Assets
Long-lived asset impairment	$ 0.0

Cash Equivalents and Investments - Tabular Disclosure - Total (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Cash Equivalents and Investments
Available-for-sale, Unrealized Gain		$ 14
Available-for-sale, Unrealized Losses		(32)
Total, Amortized Cost	$ 673,625	268,236
Total, Fair Value	$ 673,356	$ 268,218
US Treasury obligations
Cash Equivalents and Investments
Average Maturity	348 days	298 days
Available-for-sale, Amortized Cost	$ 273,321	$ 216,167
Available-for-sale, Unrealized Gain		14
Available-for-sale, Unrealized Losses	(269)	(32)
Available-for-sale, Fair Value	273,052	216,149
Money market funds
Cash Equivalents and Investments
Cash equivalents, Amortized Cost	400,304	52,069
Cash equivalents, Fair Value	$ 400,304	$ 52,069

Cash Equivalents and Investments - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Unrealized loss position
Number of held securities in an unrealized loss position \| security	37	33
Unrealized loss position for less than 12 months	$ 273.1	$ 147.1
Unrealized loss position for more than 12 months	$ 0.0

Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value of Financial Instruments
Total	$ 673,356	$ 268,218
Money market funds
Fair Value of Financial Instruments
Cash equivalents, Fair Value	400,304	52,069
US Treasury obligations
Fair Value of Financial Instruments
Investments, available-for-sale	273,052	216,149
Active Markets (Level 1)
Fair Value of Financial Instruments
Total	673,356	268,218
Active Markets (Level 1) \| Money market funds
Fair Value of Financial Instruments
Cash equivalents, Fair Value	400,304	52,069
Active Markets (Level 1) \| US Treasury obligations
Fair Value of Financial Instruments
Investments, available-for-sale	$ 273,052	$ 216,149

Fair Value of Financial Instruments - Fair Value of Warrant Liability (Details) - Warrant $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Fair value liabilities measured on recurring basis unobservable input reconciliation
Beginning balance	$ 365
Issuance of warrant at fair value	0
Change in fair value	445
Reclassification of fair value to additional paid-in capital	(810)
Ending Balance	$ 0

Restricted Cash (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Apr. 28, 2017 ft²	Dec. 31, 2016 USD ($)
Restricted Cash
Restricted cash included in long-term assets	$ 4,555		$ 1,267
Cambridge Massachusetts, 38 Sidney Street and 45 Sidney Street Leases
Restricted Cash
Restricted cash included in long-term assets	4,600		1,300
Office and Laboratory Space in Cambridge Massachusetts, 45 Sidney Street
Restricted Cash
Restricted cash included in long-term assets	3,500
Area leased (in square feet) \| ft²		99,833
Security deposit reduction amount subject to satisfaction of specified terms and conditions	1,000
Restricted by bank
Restricted Cash
Restricted Cash and Cash Equivalents	$ 4,800		$ 1,300

Property and Equipment, Net - Estimated Useful Lives (Details)	12 Months Ended
	Dec. 31, 2017
Lab equipment
Property and Equipment, Net
Estimated Useful Life (Years)	P5Y
Furniture and fixtures
Property and Equipment, Net
Estimated Useful Life (Years)	P4Y
Computer equipment
Property and Equipment, Net
Estimated Useful Life (Years)	P3Y
Software
Property and Equipment, Net
Estimated Useful Life (Years)	P3Y

Property and Equipment, Net - Property and Equipment and Related Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment, Net
Gross property and equipment	$ 29,007	$ 9,456
Less: accumulated depreciation and amortization	(4,644)	(3,268)
Total property and equipment, net	24,363	6,188
Lab equipment
Property and Equipment, Net
Gross property and equipment	3,604	3,059
Furniture and fixtures
Property and Equipment, Net
Gross property and equipment	837	784
Computer equipment
Property and Equipment, Net
Gross property and equipment	624	768
Leasehold improvements
Property and Equipment, Net
Gross property and equipment	4,681	4,673
Software
Property and Equipment, Net
Gross property and equipment	263	$ 172
Construction-in-progress
Property and Equipment, Net
Gross property and equipment	$ 18,998

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses
External research and development	$ 17,880	$ 5,696
Employee compensation	5,765	4,118
Professional fees and other	2,467	1,697
Interest	197	212
Property and equipment costs	4,176	23
Severance	56
Total accrued expenses	$ 30,541	$ 11,746

Collaborations (Details) $ in Thousands		1 Months Ended		3 Months Ended									12 Months Ended
Collaborations (Details) $ in Thousands	Jul. 26, 2017 USD ($)	Mar. 31, 2016 USD ($) item	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) item	Jun. 30, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaborations
Collaboration revenue				$ 1,628	$ 8,068	$ 5,890	$ 5,840	$ 7,691	$ 6,160	$ 7,065	$ 6,856		$ 21,426	$ 27,772	$ 11,400
Roche Agreement
Collaborations
Non-refundable upfront payment received		$ 45,000					$ 45,000
Number of deliverables \| item		5
Collaboration revenue													5,200	4,500
Roche Agreement \| Maximum
Collaborations
Number of potential collaboration programs \| item		5
Number of collaboration programs to select targets \| item		2
Total eligible contingent option fees and milestone payments		$ 965,000									$ 965,000
Eligible option fees and milestones prior to licensing for all potential collaboration programs		$ 215,000
Roche Agreement \| Exercise Of License Option Product Commercially Sold
Collaborations
Written termination notice period		180 days
Roche Agreement \| Roche
Collaborations
Number of option rights to obtain exclusive license \| item		5
Roche Agreement \| Roche \| Prior To Exercise License Right Option
Collaborations
Written termination notice period		120 days
Roche Agreement \| Roche \| Exercise Of License Right Option \| Maximum
Collaborations
Number of collaboration programs with exclusive commercialization rights for licensed products \| item		3
Roche Agreement \| Roche \| Exercise Of License Option Product Not Commercially Sold
Collaborations
Written termination notice period		120 days
Alexion
Collaborations
Collaborative partner's funding responsibility of research and development costs incurred under the research plan (as a percent)			100.00%
Non-refundable upfront payment received	$ 18,800		$ 15,000										15,000	15,000
Payment receivable upon achievement of milestone			250,000
Number of deliverables \| item											3
Refund provisions		$ 0									$ 0
Achieved milestone payment received which will be recognized over period of performance												$ 1,800		1,800
Non-substantive milestone payment previously													1,800	1,800
Revenue recognized from milestone			$ 3,800										2,000
Collaboration revenue													16,200	23,300
Reimbursable research and development costs recognized													9,500	$ 14,600
Reimbursable research and development costs received													$ 12,700

Term Loan - General Information (Details) - Secured Debt $ in Millions	1 Months Ended
	Nov. 30, 2014 USD ($)	May 31, 2013 USD ($) tranche	Dec. 31, 2017
Term Loan
Percentage fee payable on total loan advances at end of loan (as a percent)			4.00%
Percentage fee payable on outstanding principal in event of prepayment of loan advances, low end of range (as a percent)			1.00%
Percentage fee payable on outstanding principal in event of prepayment of loan advances, high end of range (as a percent)			2.00%
2013 Term Loan
Term Loan
Face amount		$ 5.0
Number of tranches \| tranche		3
2013 Term Loan \| Prime Rate
Term Loan
Fixed rate based on percentage points added to base rate (as a percent)		2.00%
2014 Term Loan
Term Loan
Face amount	$ 5.0
Proceeds from term loan	$ 5.0

Term Loan - Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Future minimum payments
2018	$ 1,528
Total	$ 1,528

Warrants - General Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	May 05, 2015	Dec. 31, 2017	Dec. 31, 2015	Nov. 30, 2014	May 31, 2013
Warrants
Change in fair value of warrant liability			$ 445
Series A and Series B Warrants
Warrants
Warrants life		10 years
Series A Warrants
Warrants
Number of shares that can be purchased by the warrants issued					150,000
Exercise price of warrants issued (in dollars per share)	$ 5.50				$ 1.00
Number of common shares that became exercisable from warrants	27,272
Series B Warrants
Warrants
Number of shares that can be purchased by the warrants issued				83,333
Exercise price of warrants issued (in dollars per share)	$ 6.60			$ 1.20
Number of common shares that became exercisable from warrants	15,151

Warrants - Valuation Assumptions (Details)	May 05, 2015 $ / shares
Series A Warrants
Weighted-average assumptions
Fair value of underlying instrument	$ 20.82
Expected volatility	91.58%
Expected term (in years)	8 years 1 month 6 days
Risk-free interest rate	2.06%
Expected dividend yield	0.00%
Series B Warrants
Weighted-average assumptions
Fair value of underlying instrument	$ 20.82
Expected volatility	87.75%
Expected term (in years)	9 years 6 months
Risk-free interest rate	2.19%
Expected dividend yield	0.00%

Warrants - Additional Information (Details) - USD ($) $ in Millions		12 Months Ended
	May 13, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 05, 2015
Series A and Series B Warrants
Warrants
Fair value of warrants					$ 0.8
Series A Warrants
Warrants
Stock issued upon exercise of warrants	21,281
Number of warrants cancelled	5,991
Series B Warrants
Warrants
Stock issued upon exercise of warrants	11,157
Number of warrants cancelled	3,994
Maximum \| Series A and Series B Warrants
Warrants
Interest expense		$ 0.1	$ 0.1	$ 0.1

Stock Awards - 2015 Stock Option and Incentive Plan (Details) - 2015 Stock Option and Incentive Plan - shares			12 Months Ended
	Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2017	Apr. 08, 2015
Stock Awards
Initial shares of common stock authorized for issuance of stock awards				1,460,084
Increase in number of shares available for grant (as a percent)			4.00%
Increase in number of shares available for grant	1,743,101	1,325,019
Number of shares available for grant			1,769,028

Stock Awards - Awards - General Information (Details) - 2015 Stock Option and Incentive Plan	12 Months Ended
	Dec. 31, 2017
Stock Awards
Award vesting period	4 years
Cliff vesting period for new employees	1 year
Award expiration period	10 years

Stock Awards - Unvested Restricted Stock - Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Shares
Unvested at beginning of period (in shares)	2,125
Vested (in shares)	(2,125)
Repurchased (in shares)	0
Unvested at end of period (in shares)	0
Weighted-Average Grant Date Fair Value
Unvested at beginning or period (in dollars per share) \| $ / shares	$ 1.25
Vested (in dollars per share) \| $ / shares	1.25
Unvested at end of period (in dollars per share) \| $ / shares	$ 0.00

Stock Awards - Stock Options - Activity (Details) - Employee Stock Options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Shares
Outstanding at beginning of period (in shares)	2,622,741
Granted (in shares)	1,212,972
Exercised (in shares)	(426,085)
Canceled (in shares)	(105,462)
Outstanding at end of period (in shares)	3,304,166	2,622,741
Weighted-Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 11.67
Granted (in dollars per share)	42.62
Exercised (in dollars per share)	11.47
Canceled (in dollars per share)	30.50
Outstanding at end of period (in dollars per share)	$ 22.45	$ 11.67
Additional disclosures
Shares - Exercisable (in shares)	1,432,958
Weighted-Average Exercise Price - Exercisable (in dollars per share)	$ 11.63
Remaining Contractual Life - Outstanding	8 years 15 days	8 years 3 months 18 days
Remaining Contractual Life - Exercisable	7 years 4 months 2 days
Aggregate Intrinsic Value - Outstanding	$ 174,985	$ 44,025
Aggregate Intrinsic Value - Exercisable	$ 91,389

Stock Awards - Weighted Average Assumptions - Tabular Disclosure (Details) - Employee Stock Options	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions used in estimating the fair value of stock options granted
Risk-free interest rate	2.07%	1.55%	1.66%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years	6 years	6 years
Expected stock price volatility	74.58%	75.94%	85.43%

Stock Awards - Weighted Average Assumptions - Additional Information (Details) - Employee Stock Options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Additional disclosures
Weighted-average grant date fair value of options granted	$ 28.04	$ 13.06	$ 8.55
Total intrinsic value of options exercised	$ 17.8	$ 3.1	$ 6.8

Stock Awards - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total stock based compensation expense
Total stock-based compensation expense	$ 12,523	$ 6,140	$ 5,180
Research and development
Total stock based compensation expense
Total stock-based compensation expense	6,296	2,674	2,148
General and administrative
Total stock based compensation expense
Total stock-based compensation expense	$ 6,227	$ 3,466	$ 3,032

Stock Awards - Unrecognized Compensation Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Stock Awards
Total unrecognized compensation cost related to non-vested stock option awards	$ 35.2
Weighted-average period over which unrecognized compensation cost will be recognized	2 years 4 months 28 days

Stock Awards - Employee Stock Purchase Plan (Details) - Employee Stock - 2015 Employee Stock Purchase Plan - shares	1 Months Ended	12 Months Ended
	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2017
Stock Awards
Number of common shares reserved for future issuance	243,347
Annual increase for common stock for issuance (as a percent)	1.00%
Increase of common shares reserved for future issuance				435,775	331,254
ESPP shares issued during period		16,703	23,325

Net Loss per Share - Anti-dilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	3,304,166	2,624,866	1,933,297
Stock options
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	3,304,166	2,622,741	1,802,802
Restricted Stock
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share		2,125	130,495

Net Loss per Share - Weighted Average Number of Common Shares (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Loss per Share
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic	37,792,929	27,491,669	18,235,614
Weighted-average number of common shares used in net loss per share applicable to common stockholders - diluted	37,792,929	27,491,669	18,235,614

Convertible Preferred Stock - Conversion (Details) - Common Stock - shares	1 Months Ended	12 Months Ended
	May 31, 2015	Dec. 31, 2015
Subsidiary, Sale of Stock [Line Items]
Conversion of preferred stock into common stock (in shares)		15,467,479
IPO
Subsidiary, Sale of Stock [Line Items]
Conversion of preferred stock into common stock (in shares)	15,467,479

Convertible Preferred Stock - Preferred Stock (Details)	Dec. 31, 2017 series shares	Dec. 31, 2016 series shares	May 31, 2015 series shares
Preferred Stock Disclosures
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000	5,000,000
Number of series of preferred stock, minimum \| series	1	1	1
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0

Income Taxes - Reconciliation (Details)	12 Months Ended
Income Taxes - Reconciliation (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
A reconciliation of the U.S. statutory income tax rate to the Company’s effective tax rate
Federal income tax (benefit) at statutory rate	34.00%	34.00%	34.00%
Permanent differences	0.49%	(2.74%)	(3.30%)
Federal research and development credits	0.97%	1.02%	1.09%
Federal orphan drug credits	6.40%	6.63%	0.55%
State income tax, net of federal benefit	5.40%	4.81%	4.72%
Other	0.83%	(0.78%)	0.56%
Deferred rate change	(27.58%)
Change in valuation allowance	(20.54%)	(42.94%)	(37.62%)
Effective income tax rate	(0.03%)	0.00%	0.00%

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 78,829	$ 69,443	$ 48,291
Research and development credit carryforwards	5,950	3,833	2,829
Orphan drug credit carryforwards	14,574	5,095	289
Accrued expenses and other	4,660	2,914	1,374
Deferred revenue	9,664	2,627
Deferred lease incentive	4,462	1,324	1,551
Deferred rent	1,142	366	331
Total gross deferred tax asset	119,281	85,602	54,665
Deferred tax liability	(4,801)	(1,535)	(1,702)
Debt discount	(2)	(14)	(39)
Valuation allowance	(114,478)	(84,053)	(52,924)
Net deferred tax asset	$ 0	$ 0	$ 0

Income Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation allowance
Valuation allowance	$ 114,478	$ 84,053	$ 52,924
Change in valuation allowance	$ 30,400	$ 31,100	$ 19,800

Income Taxes - Tax Credit Carryforwards (Details) $ in Millions	Dec. 31, 2017 USD ($)
Research and Development Tax Credit \| Federal
Tax Credit Carryforwards
Tax credit carryforwards	$ 4.2
Research and Development Tax Credit \| State
Tax Credit Carryforwards
Tax credit carryforwards	2.1
Orphan Drug Tax Credit \| Federal
Tax Credit Carryforwards
Tax credit carryforwards	14.6
Investment Tax Credit \| State
Tax Credit Carryforwards
Tax credit carryforwards	$ 0.1

Income Taxes - Tax Cuts and Jobs Act (Details)		12 Months Ended
Income Taxes - Tax Cuts and Jobs Act (Details)	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
U.S. federal corporate tax rate (as a percent)		34.00%	34.00%	34.00%
Internal Revenue Service (IRS)
Income Taxes
U.S. federal corporate tax rate (as a percent)		35.00%
Internal Revenue Service (IRS) \| Forecast
Income Taxes
U.S. federal corporate tax rate (as a percent)	21.00%

Commitments - General Information (Details) $ in Thousands	Oct. 01, 2017 USD ($) period	Feb. 12, 2015 USD ($) ft²	Dec. 31, 2017 USD ($)	Apr. 28, 2017 USD ($) ft²
Operating Leases
Total commitment under the operating lease			$ 121,083
Corporate Headquarters Lease
Operating Leases
Area leased (in square feet) \| ft²		38,500
Extension period of lease term		5 years
Total commitment under the operating lease		$ 17,800
Lease term		7 years
Base annual rent, initial		$ 2,300
Base annual rent, maximum		2,800
Allowance for leasehold improvements		4,300
Security deposit included in restricted cash		$ 1,300
Office and Laboratory Space in Cambridge Massachusetts, 45 Sidney Street
Operating Leases
Area leased (in square feet) \| ft²				99,833
Number of extensions \| period	2
Extension period of lease term	5 years
Base annual rent, initial	$ 7,700
Base annual rent, maximum	$ 10,600
Tenant improvement allowance				$ 14,200
Security deposit included in restricted cash				$ 3,500

Commitments - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Future minimum lease payments
2018	$ 10,174
2019	10,479
2020	10,794
2021	11,118
2022	10,977
Thereafter	67,541
Total minimum lease payments	$ 121,083

Defined Contribution Benefit Plan (Details) - USD ($) $ in Millions		12 Months Ended
	Sep. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Benefit Plan
Employer matching contribution, percent of match (as a percent)	50.00%
Employer matching contribution, percent of employees' gross pay	6.00%
Contribution amount by entity		$ 0.5	$ 0.4	$ 0.1

Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Data
Total revenue	$ 1,628	$ 8,068	$ 5,890	$ 5,840	$ 7,691	$ 6,160	$ 7,065	$ 6,856	$ 21,426	$ 27,772	$ 11,400
Total operating expenses	51,721	46,678	40,104	34,170	29,064	23,043	25,961	22,281	172,673	100,349	63,044
Total other income (expense), net	1,066	907	802	353	110	49	2	(79)	3,128	82	(1,125)
Net loss	$ (49,027)	$ (37,703)	$ (33,412)	$ (27,977)	$ (21,263)	$ (16,834)	$ (18,894)	$ (15,504)	$ (148,119)	$ (72,495)	$ (52,769)
Net loss per share applicable to common stockholders - basic and diluted (in dollars per share)	$ (1.23)	$ (0.96)	$ (0.86)	$ (0.84)	$ (0.75)	$ (0.62)	$ (0.70)	$ (0.57)	$ (3.92)	$ (2.64)	$ (3.07)

Subsequent Events (Details) - Subsequent Event - Corporate Headquarters Lease $ in Millions	2 Months Ended
	Feb. 21, 2018 USD ($)
Operating Leases
Total commitment in sublease	$ 8.2
Sublease term	32 months

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Basis of Presentation (Details)	6 Months Ended
	Jun. 30, 2017
Common Stock Disclosures
Reverse stock split ratio	0.1818

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Segment and Geographic Information (Details)	12 Months Ended
	Dec. 31, 2017 segment
Segment and Geographic Information
Number of operating segments	1

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Recent Accounting Pronouncements (Details) $ in Millions	Dec. 31, 2017 USD ($)
Accounting Standards Update 2016-09
Recent Accounting Pronouncements
Deferred tax assets	$ 1.1

Income Taxes - Net Operating Loss Carryforwards (Details) $ in Millions	Dec. 31, 2017 USD ($)
Federal
Net Operating Loss Carryforwards
Net operating loss carryforwards	$ 288.6
State
Net Operating Loss Carryforwards
Net operating loss carryforwards	$ 288.5

Income Taxes - Uncertain Tax Positions (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Uncertain tax positions
Accrued interest related to uncertain tax positions	$ 0.0	$ 0.0