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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2015	Feb. 29, 2016	Jun. 30, 2015
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, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 389,520,000
Entity Common Stock, Shares Outstanding		27,201,271
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 162,707	$ 47,240
Restricted cash	119	119
Unbilled accounts receivable	3,414
Prepaid expenses and other current assets	4,176	915
Total current assets	170,416	48,274
Property and equipment, net	6,661	1,482
Other assets	555	99
Restricted cash	1,266	70
Total assets	178,898	49,925
Current liabilities:
Accounts payable	2,455	814
Accrued expenses	6,436	3,810
Deferred rent		138
Restricted stock liability	7	298
Current portion of deferred revenue	5,898
Current portion of lease incentive obligation	578
Current portion of term loan payable	3,266	1,704
Total current liabilities	18,640	6,764
Deferred rent, net of current portion	842
Restricted stock liability, net of current portion	1	29
Warrant liability		365
Deferred revenue, net of current portion	7,742
Lease incentive obligation, net of current portion	3,370
Term loan, net of current portion	4,072	7,338
Other long term liabilities	252
Commitments (Note 12)
Stockholders' deficit:
Preferred stock
Common stock	27	2
Additional paid-in capital	278,927	2,822
Accumulated deficit	(134,975)	(82,206)
Total stockholders' equity (deficit)	143,979	(79,382)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 178,898	49,925
Series A convertible preferred stock
Current liabilities:
Convertible preferred stock		39,958
Series B convertible preferred stock
Current liabilities:
Convertible preferred stock		24,985
Series C convertible preferred stock
Current liabilities:
Convertible preferred stock		$ 49,868

Balance (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
Preferred Stock Disclosures
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common Stock Disclosures
Common stock, par value (in NIS dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	27,196,053	2,218,652
Common Stock, Shares, Outstanding	27,065,558	1,626,738
Series A convertible preferred stock
Temporary Equity Disclosures
Convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	40,150,000
Convertible preferred stock, shares issued	0	40,000,000
Convertible preferred stock, shares outstanding	0	40,000,000
Series B convertible preferred stock
Temporary Equity Disclosures
Convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	20,999,996
Convertible preferred stock, shares issued	0	20,916,663
Convertible preferred stock, shares outstanding	0	20,916,663
Series C convertible preferred stock
Temporary Equity Disclosures
Convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	24,154,589
Convertible preferred stock, shares issued	0	24,154,589
Convertible preferred stock, shares outstanding	0	24,154,589

Statements of Operations - USD ($) $ in Thousands	12 Months Ended
Statements of Operations - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Collaboration revenue	$ 11,400
Operating expenses:
Research and development	48,588	$ 31,844	$ 15,928
General and administrative	14,456	7,890	5,072
Total operating expenses	63,044	39,734	21,000
Other income (expense):
Other income (expense), net	(429)	(98)	226
Interest expense	(696)	(453)	(138)
Total other income (expense)	(1,125)	(551)	88
Net loss	(52,769)	(40,285)	(20,912)
Convertible preferred stock dividends	(3,153)	(5,765)	(2,870)
Net loss applicable to common stockholders	$ (55,922)	$ (46,050)	$ (23,782)
Net loss per share applicable to common stockholders - basic and diluted	$ (3.07)	$ (32.41)	$ (23.43)
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted	18,235,614	1,420,518	1,015,000

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	Retained Earnings (Accumulated Deficit)	Total
Beginning Balance at Dec. 31, 2012	$ 24,970
Beginning Balance (in shares) at Dec. 31, 2012	25,000,000
Increase (Decrease) in Convertible Preferred Stock
Issuance of convertible preferred stock value, net of issuance costs	$ 14,988
Issuance of convertible preferred stock (in shares)	15,000,000
Ending Balance at Dec. 31, 2013	$ 39,958
Ending Balance (in shares) at Dec. 31, 2013	40,000,000
Beginning Balance at Dec. 31, 2012				$ 1	$ 92	$ (21,009)	$ (20,916)
Beginning Balance (in shares) at Dec. 31, 2012				720,031
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plan					23		23
Issuance of common stock under stock plan (in shares)				501,299
Stock-based compensation expense					351		351
Net loss						(20,912)	(20,912)
Ending Balance at Dec. 31, 2013				$ 1	466	(41,921)	(41,454)
Ending Balance (in shares) at Dec. 31, 2013				1,221,330
Increase (Decrease) in Convertible Preferred Stock
Issuance of convertible preferred stock value, net of issuance costs		$ 24,985	$ 49,868
Issuance of convertible preferred stock (in shares)		20,916,663	24,154,589
Ending Balance at Dec. 31, 2014	$ 39,958	$ 24,985	$ 49,868
Ending Balance (in shares) at Dec. 31, 2014	40,000,000	20,916,663	24,154,589
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plan				$ 1	30		31
Issuance of common stock under stock plan (in shares)				405,408
Stock-based compensation expense					2,326		2,326
Net loss						(40,285)	(40,285)
Ending Balance at Dec. 31, 2014				$ 2	2,822	(82,206)	(79,382)
Ending Balance (in shares) at Dec. 31, 2014				1,626,738
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)
Ending Balance (in shares) at Dec. 31, 2015	0	0	0
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, net of issuance costs				$ 9	154,743		154,752
Initial public 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

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Series A convertible preferred stock
Share price (in dollars per share)		$ 1.00
Stock issuance cost		$ 12
Series B convertible preferred stock
Share price (in dollars per share)	$ 1.20
Stock issuance cost	$ 115
Series C convertible preferred stock
Share price (in dollars per share)	$ 2.07
Stock issuance cost	$ 131

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (52,769)	$ (40,285)	$ (20,912)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	948	622	501
Noncash interest expense	109	85	26
Change in fair value of warrant liability	445	100	(4)
Stock-based compensation	5,180	2,326	351
Changes in assets and liabilities:
Unbilled accounts receivable	(3,414)
Prepaid expenses and other current assets	(1,482)	(469)	16
Other assets	(552)		20
Accounts payable	1,391	(512)	541
Accrued expenses	1,956	2,873	550
Deferred revenue	13,641
Deferred rent	2,871	(140)	(114)
Net cash used in operating activities	(31,676)	(35,400)	(19,025)
Investing activities
Purchases of property and equipment	(4,883)	(700)	(257)
Restricted cash	(1,196)
Net cash used in investing activities	(6,079)	(700)	(257)
Financing activities
Proceeds from term loan		7,000	3,000
Principal payments on loan payable	(1,806)	(750)
Proceeds from IPO, net of commissions and underwriting discounts	156,815
Debt issuance costs		(18)	(48)
Payment of offering costs	(2,046)
Proceeds from issuance of common stock, net of repurchases	259	268	53
Net cash provided by financing activities	153,222	81,353	17,992
Net increase in cash and cash equivalents	115,467	45,253	(1,290)
Cash and cash equivalents at beginning of period	47,240	1,987	3,277
Cash and cash equivalents at end of period	162,707	47,240	1,987
Supplemental cash flow information
Cash paid for interest	452	215	53
Conversion of convertible preferred stock into common stock	114,808
Reclassification of warrant liability to additional paid-in-capital	810
Property and equipment purchases incurred but unpaid at period end	$ 1,244
Issuance of warrants in connection with term loan		145	119
Series A convertible preferred stock
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs			$ 14,987
Series B convertible preferred stock
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs		24,985
Series C convertible preferred stock
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 49,868

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2015
Nature of Business
Nature of Business	1. Nature of Business Blueprint Medicines Corporation (the Company), a Delaware corporation formed on October 14, 2008, is a biopharmaceutical company focused on improving the lives of patients with genomically defined diseases driven by abnormal kinase activation. The Company’s approach is to systematically and reproducibly identify kinases that are drivers of diseases in genomically defined patient populations and to craft drug candidates with therapeutic windows that may provide significant and durable clinical response to 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, 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, at a price to the public of $18.00 per share. The Company received net proceeds of $154.8 million after deducting underwriting discounts and commissions and offering costs payable by the Company.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2015

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

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).

The accompanying consolidated financial statements include the accounts of Blueprint Medicines Corporation 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.

In connection with preparing for its IPO, the Company effected a 1‑for‑5.5 reverse stock split of the Company’s common stock. The reverse stock split became effective on April 10, 2015. 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. Upon the closing of the IPO in May 2015, all of the Company’s outstanding convertible preferred stock automatically converted into 15,467,479 shares of common stock, and warrants exercisable for convertible preferred stock automatically converted into warrants exercisable for 42,423 shares of common stock. In addition, the Company is authorized to issue 120,000,000 shares of common stock and 5,000,000 shares of preferred stock. The significant increase in shares outstanding in the year ended December 31, 2015 is expected to impact the year-over-year comparability of the Company’s net loss per share calculations until the second quarter of 2016.

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.

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.

The Company’s revenue is currently generated through its collaboration agreement with Alexion Pharma Holding (Alexion). The terms of this agreement contain multiple elements, or deliverables, including an exclusive license granted by the Company to Alexion to research, develop, manufacture and commercialize the licensed products and the compounds in the field in the territory, as well as research and development activities to be performed by the Company on behalf of Alexion related to the licensed product candidates. In addition, the terms of this agreement include payments to the Company of one or more of the following: a nonrefundable, upfront payment; contingent milestone payments related to specified pre-clinical milestones, development milestones and sales-based commercial milestones; fees for research and development services rendered; and royalties on commercial sales of licensed product candidates, if any. To date, the Company has received the upfront payment, payments for the achievement of certain pre-clinical milestones under the Alexion agreement and payments for certain research and development services. The Company is eligible to earn additional milestone payments under the Alexion agreement. The Company has not earned royalty revenue as a result of product sales. See Note 13 for additional information on this agreement.

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 agreement with Alexion does 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, 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. For the years ended December 31, 2015, 2014 and 2013, comprehensive loss was equal to net loss.

Cash Equivalents

Cash equivalents are highly liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. These assets include an investment in a money market fund that invests in U.S. treasury obligations. Cash equivalents consist of the following at December 31, 2015 and December 31, 2014 (in thousands):


	December 31,		December 31,
	2015		2014
Money market fund	$	162,707	$	47,240

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, 2015, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2015		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Money market funds, included in cash equivalents	$	162,707	$	162,707	$	—	$	—

Financial instruments measured at fair value as of December 31, 2014, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2014		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Money market funds, included in cash equivalents	$	47,240	$	47,240	$	—	$	—
Financial Liabilities
Preferred stock warrants		(365)		—		—		(365)

At December 31, 2015 and December 31, 2014, all of the Company’s cash equivalents were comprised of a money market account, the fair value of which is valued using Level 1 inputs. 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 7). 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 and 2014 (in thousands):


	Year Ended
	December 31,
	2015		2014
Beginning balance	$	365	$	119
Issuance of warrant at fair value		—		146
Change in fair value		445		100
Reclassification of fair value to additional paid-in capital		(810)		—
Ending balance	$	—	$	365

Deferred Offering Costs

The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After completion of the IPO in May 2015, $2.1 million of these costs were recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital. As of December 31, 2014, the Company recorded $0.1 million of deferred offering costs, included in other assets in the accompanying balance sheet, in contemplation of its IPO.

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 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. As of December 31, 2015 and 2014, the Company had prepaid expenses of approximately $0.4 million and $0.1 million, respectively, related to nonrefundable advance payments to vendors.

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, 2015.

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 completion of a liquidation event, including the IPO, at which time the warrant liability was reclassified to stockholders’ equity. The warrant liability totaled $0.4 million at December 31, 2014 (see Note 7).

Stock‑Based Compensation Expense

The Company expenses the fair value of employee stock awards net of estimated forfeitures on a straight‑line basis over the requisite service period, which generally is the vesting period. Compensation cost for restricted stock awards issued to employees is measured using the grant date intrinsic value of the award, net of estimated forfeitures, 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 the same representative companies until a sufficient amount of historical information regarding the volatility of its own share price becomes available or until circumstances change, such that the identified entities are no longer representative companies. In the latter case, more suitable, similar entities whose share prices are publicly available would be utilized in the calculation;

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 life 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.

The amount of stock‑based compensation expense recognized during a period is based on the fair value of the portion of the awards that are ultimately expected to vest. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The term “forfeitures” is distinct from “cancellations” or “expirations” and represents only the unvested portion of the surrendered option. The Company evaluates its forfeiture rate at each reporting period. Ultimately, the actual expense recognized over the vesting period will be for only those options that vest.

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 and accounts receivable.

The Company maintains its cash and cash equivalents in a custodian account at a high quality financial institution, 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.

Recent Accounting Pronouncements

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 and most industry-specific guidance. 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; early adoption in 2017 is permitted. Companies have the option of applying this new guidance retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application. The Company is in the process of evaluating the new guidance and determining the expected effects of the adoption of this standard on its consolidated financial statements.

On April 7, 2015, the FASB, issued ASU 2015-03, Simplifying the Presentation of Debt Issuance Costs (ASU 2015-03),which requires debt issuance costs to be presented in the balance sheet as a direct deduction from the carrying value of the associated debt liability, consistent with the presentation of a debt discount. ASU 2015-03 will be effective for the Company on January 1, 2016, with early adoption permitted. ASU 2015-03 will be applied on a retrospective basis. The Company has assessed and determined the impact of the adoption of ASU 2015-03 on its financial statements will be immaterial.

In 2014, the FASB issued new guidance on management’s responsibility in evaluating whether or not there is substantial doubt about a company’s ability to continue as a going concern within one year from the date the financial statements are issued each reporting period. This new accounting guidance is effective for annual periods ending after December 15, 2016. Early adoption is permitted. The new accounting standard will impact the disclosure in the Company’s financial statements.

Restricted Cash	12 Months Ended
Restricted Cash	Dec. 31, 2015
Restricted Cash
Restricted Cash	3. Restricted Cash At December 31, 2015 and 2014, $1.4 million and $0.2 million, respectively, of the Company’s cash is restricted by a bank. As of December 31, 2015 and 2014, $0.1 million of the restricted cash was included in current assets as collateral for a stand‑by letter of credit issued by the Company to its landlord in connection with the lease of the Company’s corporate headquarters, which ended in October 2015. As of December 31, 2015, $1.3 million of restricted cash was included in long-term assets on the Company’s balance sheet related to a security deposit for the lease agreement entered on February 12, 2015 for the Company’s new corporate headquarters. In addition, $0.1 million of restricted cash was included in long‑term assets related to the Company’s corporate credit card agreement as of December 31, 2014.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2015

Property and Equipment, Net

Property and Equipment, Net

4. Property and Equipment, Net

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


	Estimated
	Useful Life	December 31
	(Years)	2015		2014
Lab equipment	5	$	2,712	$	2,000
Furniture and fixtures	4		632		326
Computer equipment	3		615		369
Leasehold improvements	Term of lease		4,612		191
Software	3		167		142
			8,738		3,028
Less: accumulated depreciation and amortization			(2,077)		(1,546)
Total property and equipment, net		$	6,661	$	1,482

Depreciation expense for the years ended December 31, 2015, 2014 and 2013 was $0.9 million, $0.6 million and $0.5 million, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2015

Accrued Expenses

Accrued Expenses

5. Accrued Expenses

Accrued expenses consist of the following (in thousands):


	December 31,		December 31,
	2015		2014
Employee compensation	$	2,731	$	623
External research and development		1,471		2,034
Property and equipment costs		994		—
Professional fees and other		1,035		386
Consulting		165		216
Interest		34		150
Severance		6		330
Deferred offering costs		—		71
	$	6,436	$	3,810

Term Loan

12 Months Ended

Dec. 31, 2015

Term Loan

Term Loan

6. 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 June 2013, the Company drew the first loan advance of $1.0 million under the 2013 Term Loan and was required to make interest‑only payments until April 1, 2014, and consecutive monthly payments of principal, plus accrued interest, over the remaining term through March 2017. In September 2013, the Company drew the second loan advance of $2.0 million under the 2013 Term Loan and was required to make interest‑only payments until April 1, 2014, and consecutive monthly payments of principal, plus accrued interest, over the remaining term through March 2017. In June 2014, the Company drew the remaining $2.0 million advance under the 2013 Term Loan and was required to make interest‑only payments until January 1, 2015, and consecutive monthly payments of principal, plus accrued interest, over the remaining term through December 2017. 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.

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 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 and long‑term 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, 2015, are as follows (in thousands):


2016		3,333
2017		2,583
2018		1,528
Total	$	7,444

Warrants

12 Months Ended

Dec. 31, 2015

Warrants

Warrants

7. 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 were re‑measured to the‑then current fair value at each balance sheet date. Re‑measurement gains or losses were recorded in other income (expense) in the statements of operations. 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 used the following weighted‑average assumptions in its Black‑Scholes option pricing model:


	Series A Warrant						Series B Warrant
	December 31,						December 31,
	2014			Issuance			2014			Issuance
Fair value of underlying instrument	$	1.69		$	1.00		$	1.97		$	1.97
Expected volatility		89.98	%		80.70	%		87.38	%		87.18	%
Expected term (in years)		8.4			10.0			9.8			10.0
Risk-free interest rate		2.15	%		2.58	%		2.24	%		2.36	%
Expected dividend yield		—	%		—	%		—	%		—	%

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, 2015, 2014 and 2013.

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. The Company recorded other expense of $0.4 million in the statement of operations during the twelve months ended December 31, 2015 equal to the change in fair value of the warrants from December 31, 2014 to May 5, 2015. The Company reclassified the fair value of the warrants at May 5, 2015, of $0.8 million, 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.

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2015
Convertible Preferred Stock
Convertible Preferred Stock	8. Convertible Preferred Stock In January and September 2013, the Company issued 10,000,000 and 5,000,000 shares, respectively, of Series A Convertible Preferred Stock at a price of $1.00 per share, resulting in net proceeds of $15.0 million. In January 2014, the Company issued a total of 20,916,663 shares of Series B Convertible Preferred Stock at $1.20 per share for net proceeds of $25.0 million. In November 2014, the Company issued a total of 24,154,589 shares of Series C Convertible Preferred Stock at $2.07 per share for net proceeds of $49.9 million. Upon the closing of the IPO in May 2015, all of the Company’s outstanding convertible preferred stock automatically converted into 15,467,479 shares of common stock. In addition, upon the completion 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, 2015, no shares of preferred stock were issued or outstanding.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB TOPIC 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

+ Details

Stock Awards

12 Months Ended

Dec. 31, 2015

Stock Awards

Stock Awards

9. Stock Awards

2015 Stock Option and Incentive Plan

In March 2015 and April 2015, respectively, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan (the 2015 Plan), which became effective upon the closing of the IPO in May 2015. The Plan replaced the Company’s 2011 Stock Option and Grant Plan, as amended (the 2011 Plan). Any options or awards outstanding under the 2011 Plan remain outstanding and effective. The 2015 Plan provides the Company flexibility to use various equity‑based incentive and other awards as compensation tools to motivate its workforce. These tools include incentive stock options (ISO), nonstatutory stock options (NSO), 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, which 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, 2016, the number of shares reserved for issuance under the 2015 Plan was increased by 1,087,842 shares. 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, 2015, there were 1,439,717 shares available for future grant under the 2015 Plan. ISOs may not be granted at less than fair value on the date of the grant. The exercise price of ISOs granted to an employee, who at the time of grant is a 10% shareholder, may not be less than 110% of the fair value on the date of grant.

Terms of restricted stock awards and stock option agreements, including vesting requirements, are determined by the board of directors or compensation committee of the board of directors, subject to the provisions of the applicable plan. 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. For options and restricted stock awards granted prior to the IPO, the exercise price equaled the estimated fair value of the common stock as determined by the board of directors on the date of grant. The dates of the Company’s contemporaneous valuations have not always coincided with the dates of the stock option grants. For financial reporting purposes, the Company performed common stock valuations with the assistance of a third‑party specialist, as of January 6, 2014, July 30, 2014, November 10, 2014, February 1, 2015 and March 1, 2015 to determine stock‑based compensation expense. Upon completion of the IPO, the fair value of the common stock on the grant date was based on the closing price of the stock on the date of grant.

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


		Weighted-Average
		Grant Date
	Shares	Fair Value
Unvested at December 31, 2014	425,279	$	0.46
Granted	—		—
Vested	(271,958)		0.39
Repurchased	(22,826)		0.50
Unvested at December 31, 2015	130,495		0.60

The Company has granted restricted stock to non‑employees which contain both performance‑based and service‑based vesting criteria. Stock‑based compensation expense associated with these performance‑based awards is recognized if the performance condition is considered probable of achievement using management’s best estimates. In the year ended December 31, 2014 management concluded that the milestones associated with 181,818 shares of performance‑based restricted stock were probable of achievement, and the Company began to record stock‑based compensation expense using the accelerated attribution method, accordingly. The Company recorded $1.4 million and $0.9 million of stock-based compensation expense for non‑employee performance‑based awards in the years ended December 31, 2015 and 2014, respectively, related to the achievement of both milestones.

The total fair value of restricted stock that vested during the years ended December 31, 2015, 2014 and 2013 was $4.9 million, $1.5 million and $0.5 million, respectively.

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


		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(2)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2014	1,501,912	$	2.02	9.42	$	7,704
Granted	716,573		11.96
Exercised	(289,237)		1.95
Canceled	(126,446)		3.50
Outstanding at December 31, 2015	1,802,802	$	5.88	8.76	$	37,008
Exercisable at December 31, 2015	430,939	$	3.50	8.56	$	9,847
Vested and expected to vest at December 31, 2015(1)	1,757,456	$	5.86	8.76	$	36,103

(1)

Represents the number of vested options as of December 31, 2015, plus the number of unvested options expected to vest as of December 31, 2015 based on a forfeiture rate of 2.5%.

(2)

Intrinsic value represents the amount by which the fair market value as of December 31, 2015 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,
	2015		2014		2013
Risk-free interest rate	1.66	%	1.92	%	1.87	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.0		6.1		6.1
Expected stock price volatility	85.43	%	92.99	%	88.96	%

The weighted‑average grant date fair value of options granted in the years ended December 31, 2015, 2014 and 2013 was $8.55, $3.46 and $1.18, respectively. The total intrinsic value of options exercised in the years ended December 31, 2015 and 2014 was $6.8 million and $0.1 million, respectively. There were no options exercised in the year ended December 31, 2013.

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


	Year Ended
	December 31,
	2015		2014		2013
Research and development	$	2,148	$	1,052	$	224
General and administrative		3,032		1,274		127
Total stock-based compensation expense	$	5,180	$	2,326	$	351

At December 31, 2015, there was $8.1 million of total unrecognized compensation cost related to non-vested stock awards, which is expected to be recognized over a weighted‑average period of 2.36 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 March 2015 and April 2015, respectively, 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, which will be cumulatively increased on January 1 of each calendar year 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, 2016, the number of shares reserved for issuance under the 2015 ESPP was increased by 271,960 shares. The first offering period under the 2015 Employee Stock Purchase Plan commenced on December 1, 2015.

Net Loss per Share

12 Months Ended

Dec. 31, 2015

Net Loss per Share

Net Loss per Share

10. 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,
	2015	2014	2013
Convertible preferred stock	—	15,467,479	7,272,726
Warrants	—	42,423	27,272
Stock options	1,802,802	1,501,912	385,250
Unvested restricted stock	130,495	425,279	836,712
Total	1,933,297	17,437,093	8,521,960

The weighted average number of common shares used in net loss per share applicable to common stockholders on a basic and diluted basis were 18,235,614 and 1,420,518 for the years ended December 31, 2015 and 2014, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2015

Income Taxes

Income Taxes

11. 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, 2015, 2014 and 2013:


	Year Ended
	December 31,
	2015		2014		2013
Federal income tax (benefit) at statutory rate	34.00	%	34.00	%	34.00	%
Permanent differences	(3.30)		(1.87)		0.38
Federal research and development credits	1.64		1.3		2.08
State income tax, net of federal benefit	4.72		4.93		5.16
Other	0.56		0.7		1.46
Change in valuation allowance	(37.62)		(39.06)		(43.08)
Effective income tax rate	—	%	—	%	—	%

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,
	2015		2014		2013
Deferred tax assets:
Net operating loss carryforwards	$	48,291	$	30,603	$	15,993
Research and development credit carryforwards		3,118		1,949		1,123
Deferred lease incentive		1,551		—
Accrued expenses and other		1,374		628		294
Deferred rent		331		54		109
Total gross deferred tax asset		54,665		33,234		17,519
Deferred tax liability		(1,702)		(85)		(139)
Debt discount		(39)		(74)		(41)
Valuation allowance		(52,924)		(33,075)		(17,339)
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 $52.9 million, $33.0 million and $17.3 million has been established at December 31, 2015, 2014 and 2013, respectively. The change in the valuation allowance was $19.8 million, $15.7 million and $9.0 million for the years ended December 31, 2015, 2014 and 2013, respectively. The Company has incurred net operating losses (NOL) since inception. At December 31, 2015, the Company had federal and state NOL carryforwards of $123.2 million and $121.3 million, respectively, which expire beginning in 2030. As of December 31, 2015, the Company had federal and state research and development tax credit carryforwards of $2.1 million and $1.1 million, respectively, which expire beginning in 2025. The Company had net NOLs related to stock compensation in the amount of $0.6 million that is not included in the deferred tax assets. When the excess stock-based compensation related to NOL carryover tax assets are realized, the benefit will be credited directly to equity. As of December 31, 2015, the Company had federal orphan drug credits of $0.3 million, which expire beginning in 2025.

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. As of December 31, 2015 and 2014, 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.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2015

Commitments and Contingencies Disclosure

Commitments and Contingencies

12. Commitments and Contingencies

The Company leased its prior corporate headquarters under an operating lease that expired on November 1, 2015. On February 1, 2015, the Company’s option to extend the term of the lease for an additional three‑year period expired. The Company did not exercise its option to extend the term of the lease.

On February 12, 2015, the Company entered into a lease for its current corporate headquarters 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 increases periodically until it reaches approximately $2.8 million. The current 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 consolidated balance sheets. As of December 31, 2015, the Company had other current assets of $1.3 million related to the reimbursement of leasehold improvement costs from the landlord. The lease agreement required the Company to pay a security deposit of $1.3 million, which was recorded in restricted cash on the consolidated balance sheets.

The future minimum lease payments at December 31, 2015, are as follows (in thousands):


2016	$	2,326
2017		2,396
2018		2,468
2019		2,542
2020		2,618
Thereafter		5,000
Total minimum lease payments	$	17,350

The Company records rent expense under its lease agreements on a straight line basis. For the years ended December 31, 2015, 2014, and 2013, rent expense was $1.8 million, $0.8 million, and $0.6 million, respectively.

Collaborations	12 Months Ended
Collaborations	Dec. 31, 2015
Collaborations
Collaborations	13. Collaborations Alexion In March 2015, the Company entered into a research, development and commercialization agreement with Alexion to research, develop and commercialize drug candidates for an undisclosed activated kinase target, which is the cause of a rare genetic disease. Under the terms of this agreement, the Company is responsible for research and pre-clinical development activities related to drug candidates and Alexion is responsible for all clinical development, manufacturing and commercialization activities related to drug candidates. Alexion is 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. The Company received a $15.0 million non‑refundable upfront payment in March 2015 upon execution of the agreement and is eligible to receive over $250.0 million in payments upon the successful achievement of pre‑specified pre-clinical, clinical, regulatory and commercial milestones as follows: (i) up to $6.0 million in pre-clinical milestone payments for the first licensed product, (ii) up to $83.0 million and $61.5 million in development milestone payments for the first and second licensed products, respectively, and (iii) up to $51.0 million in commercial milestone payments for each of the first and second licensed products. Alexion will 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. The royalty term for each licensed product in each country is the period commencing with first commercial sale of such licensed product in such country and ending on the later of (i) the expiration of the last‑to‑expire valid claim of specified patents covering such licensed product, (ii) the expiration of the applicable regulatory exclusivity period, and (iii) 10 or 15 years from specified commercial sales. There are no refund provisions in the agreement. Alexion has the right to terminate the Alexion agreement if the Company undergoes a change of control or becomes an affiliate of a biotechnology or pharmaceutical company, and may terminate the agreement at will upon 90 days prior written notice. The Company and Alexion have the right to terminate the agreement in the event of the other party’s uncured breach or insolvency, and in certain other circumstances agreed to by the parties The Company determined that there were three deliverables under the Alexion agreement: (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 are not separable and, accordingly, the license, undelivered research and development activities and JSC and JPT participation are 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 are expected to be delivered over the same performance period. The Company is utilizing a proportional performance model to recognize revenue under the Alexion agreement. The Company evaluated whether the milestones that may be received in connection with the agreement are substantive or non-substantive milestones. The Company concluded that the first pre-clinical milestone payment in the agreement is 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 Alexion agreement and received a $1.75 million payment from Alexion. The Company is recognizing revenues from the related milestone payment over the period of performance. The remaining non-refundable pre-clinical milestones that are expected to be achieved as a result of the Company’s efforts during the period of substantial involvement are considered substantive and are recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. The Company recognized the first substantive milestone of $0.25 million in the fourth quarter of 2015 and received payment from Alexion in the first quarter of 2016. In the first quarter of 2016, the Company recognized the second substantive milestone of $0.75 million. Milestones that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are generally achieved after the period of substantial involvement and are recognized as revenue upon achievement of the milestone, as there are no undelivered elements remaining and no continuing performance obligations, assuming all other revenue recognition criteria are met. During the year ended December 31, 2015, the Company recognized revenue under the Alexion agreement of $11.4 million, and the Company received $4.9 million related to reimbursable research and development costs under the Alexion agreement. As of December 31, 2015, the Company has recorded unbilled accounts receivable of $3.2 million related to reimbursable research and development costs under the Alexion agreement for activities performed during the fourth quarter of 2015.

Related-Party Transactions	12 Months Ended
Related-Party Transactions	Dec. 31, 2015
Related-Party Transactions
Related-Party Transactions	14. Related‑Party Transactions The Company has received consulting and management services from one of its investors, Third Rock Ventures LLC (Third Rock Ventures). The Company paid Third Rock Ventures $0.4 million for these services during each of the years ended December 31, 2014 and 2013. The Company did not receive any consulting services from Third Rock Ventures during the year ended December 31, 2015.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References + Details

Defined Contribution Benefit Plan	12 Months Ended
Defined Contribution Benefit Plan	Dec. 31, 2015
Defined Contribution Benefit Plan
Defined Contribution Benefit Plan	15. 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 year ended December 31, 2015, the Company contributed $0.1 million to the 401(k) Plan.

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2015

Selected Quarterly Financial Data (unaudited)

Selected Quarterly Financial Data (unaudited)

16. Selected Quarterly Financial Data (unaudited)

The following table contains selected quarterly financial information for 2015 and 2014. 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, 2015		June 30, 2015		September 30, 2015		December 31, 2015
		(in thousands, except per share data)
Total revenue	$	652	$	2,687	$	3,426	$	4,635
Total operating expenses		12,002		15,083		15,903		20,056
Total other expense, net		(222)		(584)		(165)		(154)
Net loss	$	(11,572)	$	(12,980)	$	(12,642)	$	(15,575)
Net loss applicable to common stockholders	$	(13,842)	$	(13,863)	$	(12,642)	$	(15,575)
Net loss per share applicable to common stockholders — basic and diluted	$	(8.23)	$	(0.81)	$	(0.47)	$	(0.58)


		Three Months Ended
		March 31, 2014		June 30, 2014		September 30, 2014		December 31, 2014
		(in thousands, except per share data)
Total operating expenses	$	6,953	$	8,199	$	10,289	$	14,293
Total other expense, net		(74)		(88)		(214)		(175)
Net loss	$	(7,027)	$	(8,287)	$	(10,503)	$	(14,468)
Net loss applicable to common stockholders	$	(8,276)	$	(9,585)	$	(11,816)	$	(16,373)
Net loss per share applicable to common stockholders — basic and diluted	$	(6.44)	$	(6.99)	$	(8.13)	$	(10.44)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2015

Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates

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)

Cash Equivalents


	December 31,		December 31,
	2015		2014
Money market fund	$	162,707	$	47,240

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.

Financial instruments measured at fair value as of December 31, 2015, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2015		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Money market funds, included in cash equivalents	$	162,707	$	162,707	$	—	$	—

Financial instruments measured at fair value as of December 31, 2014, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2014		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Money market funds, included in cash equivalents	$	47,240	$	47,240	$	—	$	—
Financial Liabilities
Preferred stock warrants		(365)		—		—		(365)


	Year Ended
	December 31,
	2015		2014
Beginning balance	$	365	$	119
Issuance of warrant at fair value		—		146
Change in fair value		445		100
Reclassification of fair value to additional paid-in capital		(810)		—
Ending balance	$	—	$	365

Deferred Offering Costs

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 life 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

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2015

Summary of Significant Accounting Policies

Schedule of cash equivalents

Cash equivalents consist of the following at December 31, 2015 and December 31, 2014 (in thousands):


	December 31,		December 31,
	2015		2014
Money market fund	$	162,707	$	47,240

Schedule of financial instruments measured at fair value

Financial instruments measured at fair value as of December 31, 2015, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2015		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Money market funds, included in cash equivalents	$	162,707	$	162,707	$	—	$	—

Financial instruments measured at fair value as of December 31, 2014, are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2014		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Money market funds, included in cash equivalents	$	47,240	$	47,240	$	—	$	—
Financial Liabilities
Preferred stock warrants		(365)		—		—		(365)

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

The following table presents activity in the preferred stock warrant liability during the year ended December 31, 2015 and 2014 (in thousands):


	Year Ended
	December 31,
	2015		2014
Beginning balance	$	365	$	119
Issuance of warrant at fair value		—		146
Change in fair value		445		100
Reclassification of fair value to additional paid-in capital		(810)		—
Ending balance	$	—	$	365

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2015

Property and Equipment, Net

Schedule of property and equipment

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


	Estimated
	Useful Life	December 31
	(Years)	2015		2014
Lab equipment	5	$	2,712	$	2,000
Furniture and fixtures	4		632		326
Computer equipment	3		615		369
Leasehold improvements	Term of lease		4,612		191
Software	3		167		142
			8,738		3,028
Less: accumulated depreciation and amortization			(2,077)		(1,546)
Total property and equipment, net		$	6,661	$	1,482

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2015

Accrued Expenses

Schedule of accrued expenses

Accrued expenses consist of the following (in thousands):


	December 31,		December 31,
	2015		2014
Employee compensation	$	2,731	$	623
External research and development		1,471		2,034
Property and equipment costs		994		—
Professional fees and other		1,035		386
Consulting		165		216
Interest		34		150
Severance		6		330
Deferred offering costs		—		71
	$	6,436	$	3,810

Term Loan (Tables)

12 Months Ended

Dec. 31, 2015

Term Loan

Schedule of principal payments on the term loan

Scheduled monthly principal payments on the term loan, as of December 31, 2015, are as follows (in thousands):


2016		3,333
2017		2,583
2018		1,528
Total	$	7,444

Warrants (Tables)

12 Months Ended

Dec. 31, 2015

Warrants

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


	Series A Warrant						Series B Warrant
	December 31,						December 31,
	2014			Issuance			2014			Issuance
Fair value of underlying instrument	$	1.69		$	1.00		$	1.97		$	1.97
Expected volatility		89.98	%		80.70	%		87.38	%		87.18	%
Expected term (in years)		8.4			10.0			9.8			10.0
Risk-free interest rate		2.15	%		2.58	%		2.24	%		2.36	%
Expected dividend yield		—	%		—	%		—	%		—	%

IPO

Warrants

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, 2015

Stock Awards

Summary of unvested restricted stock


		Weighted-Average
		Grant Date
	Shares	Fair Value
Unvested at December 31, 2014	425,279	$	0.46
Granted	—		—
Vested	(271,958)		0.39
Repurchased	(22,826)		0.50
Unvested at December 31, 2015	130,495		0.60

Summary of stock option activity

		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(2)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2014	1,501,912	$	2.02	9.42	$	7,704
Granted	716,573		11.96
Exercised	(289,237)		1.95
Canceled	(126,446)		3.50
Outstanding at December 31, 2015	1,802,802	$	5.88	8.76	$	37,008
Exercisable at December 31, 2015	430,939	$	3.50	8.56	$	9,847
Vested and expected to vest at December 31, 2015(1)	1,757,456	$	5.86	8.76	$	36,103

(1)

Represents the number of vested options as of December 31, 2015, plus the number of unvested options expected to vest as of December 31, 2015 based on a forfeiture rate of 2.5%.

(2)

Intrinsic value represents the amount by which the fair market value as of December 31, 2015 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,
	2015		2014		2013
Risk-free interest rate	1.66	%	1.92	%	1.87	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.0		6.1		6.1
Expected stock price volatility	85.43	%	92.99	%	88.96	%

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 is as follows (in thousands):


	Year Ended
	December 31,
	2015		2014		2013
Research and development	$	2,148	$	1,052	$	224
General and administrative		3,032		1,274		127
Total stock-based compensation expense	$	5,180	$	2,326	$	351

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2015

Net Loss per Share

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


	Year Ended
	December 31,
	2015	2014	2013
Convertible preferred stock	—	15,467,479	7,272,726
Warrants	—	42,423	27,272
Stock options	1,802,802	1,501,912	385,250
Unvested restricted stock	130,495	425,279	836,712
Total	1,933,297	17,437,093	8,521,960

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2015

Income Taxes

Schedule of the reconciliation of the effective tax rate


	Year Ended
	December 31,
	2015		2014		2013
Federal income tax (benefit) at statutory rate	34.00	%	34.00	%	34.00	%
Permanent differences	(3.30)		(1.87)		0.38
Federal research and development credits	1.64		1.3		2.08
State income tax, net of federal benefit	4.72		4.93		5.16
Other	0.56		0.7		1.46
Change in valuation allowance	(37.62)		(39.06)		(43.08)
Effective income tax rate	—	%	—	%	—	%

Schedule of the deferred tax assets and liabilities


	Year Ended
	December 31,
	2015		2014		2013
Deferred tax assets:
Net operating loss carryforwards	$	48,291	$	30,603	$	15,993
Research and development credit carryforwards		3,118		1,949		1,123
Deferred lease incentive		1,551		—
Accrued expenses and other		1,374		628		294
Deferred rent		331		54		109
Total gross deferred tax asset		54,665		33,234		17,519
Deferred tax liability		(1,702)		(85)		(139)
Debt discount		(39)		(74)		(41)
Valuation allowance		(52,924)		(33,075)		(17,339)
Net deferred tax asset	$	—	$	—	$	—

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2015

Commitments and Contingencies Disclosure

Schedule of future minimum lease payments

The future minimum lease payments at December 31, 2015, are as follows (in thousands):


2016	$	2,326
2017		2,396
2018		2,468
2019		2,542
2020		2,618
Thereafter		5,000
Total minimum lease payments	$	17,350

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2015

Selected Quarterly Financial Data (unaudited)

Schedule of quarterly financial information


		Three Months Ended
		March 31, 2015		June 30, 2015		September 30, 2015		December 31, 2015
		(in thousands, except per share data)
Total revenue	$	652	$	2,687	$	3,426	$	4,635
Total operating expenses		12,002		15,083		15,903		20,056
Total other expense, net		(222)		(584)		(165)		(154)
Net loss	$	(11,572)	$	(12,980)	$	(12,642)	$	(15,575)
Net loss applicable to common stockholders	$	(13,842)	$	(13,863)	$	(12,642)	$	(15,575)
Net loss per share applicable to common stockholders — basic and diluted	$	(8.23)	$	(0.81)	$	(0.47)	$	(0.58)


		Three Months Ended
		March 31, 2014		June 30, 2014		September 30, 2014		December 31, 2014
		(in thousands, except per share data)
Total operating expenses	$	6,953	$	8,199	$	10,289	$	14,293
Total other expense, net		(74)		(88)		(214)		(175)
Net loss	$	(7,027)	$	(8,287)	$	(10,503)	$	(14,468)
Net loss applicable to common stockholders	$	(8,276)	$	(9,585)	$	(11,816)	$	(16,373)
Net loss per share applicable to common stockholders — basic and diluted	$	(6.44)	$	(6.99)	$	(8.13)	$	(10.44)

Nature of Business (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	May 05, 2015	Dec. 31, 2015
Initial Public Offering
Proceeds from issuance of IPO shares		$ 156,815
Common Stock
Initial Public Offering
Stock sold (in shares)		9,367,708
IPO \| Common Stock
Initial Public Offering
Stock sold (in shares)	9,367,708
Shares issued to underwriters in IPO	1,221,874
Share price (in dollars per share)	$ 18.00
Proceeds from issuance of IPO shares	$ 154,800

Summary of Significant Accounting Policies - Stock Split and Conversion of Stock (Details)	May 05, 2015 shares	Apr. 10, 2015	Dec. 31, 2015 shares	Dec. 31, 2014 shares
Stock Split and Conversion of stock
Common stock, shares authorized			120,000,000	120,000,000
Preferred stock, shares authorized			5,000,000	5,000,000
IPO \| Common Stock
Stock Split and Conversion of stock
Reverse stock split ratio		0.1818
Shares issued upon stock conversion (in shares)	15,467,479
Number of shares that can be purchased by the warrants issued	42,423
Common stock, shares authorized	120,000,000
IPO \| Preferred Stock
Stock Split and Conversion of stock
Preferred stock, shares authorized	5,000,000

Summary of Significant Accounting Policies - Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Money market fund
Cash Equivalents
Cash equivalents	$ 162,707	$ 47,240

Summary of Significant Accounting Policies - Fair Value Measurements (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value of Financial Instruments
Preferred stock warrants		$ (365)
Money market fund
Fair Value of Financial Instruments
Cash equivalents	$ 162,707	47,240
Active Markets (Level 1) \| Money market fund
Fair Value of Financial Instruments
Cash equivalents	$ 162,707	47,240
Unobservable Inputs (Level 3)
Fair Value of Financial Instruments
Preferred stock warrants		$ (365)

Summary of Significant Accounting Policies - Fair Value Of Warrant Liability (Details) - Warrant - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair value liabilities measured on recurring basis unobservable input reconciliation
Beginning balance	$ 365	$ 119
Issuance of warrant at fair value		146
Change in fair value of warrant liability	445	100
Reclassification of fair value to additional paid-in capital	$ (810)
Ending Balance		$ 365

Summary of Significant Accounting Policies - Deferred Offering Costs (Details) - USD ($) $ in Millions	1 Months Ended
	May 31, 2015	Dec. 31, 2014
Summary of Significant Accounting Policies
Reduction of additional paid-in capital due to stock issuance costs	$ 2.1
Deferred Offering Costs		$ 0.1

Summary of Significant Accounting Policies - Other (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)
Research And Development Costs
Prepaid expense for research and development	$ 400	$ 100
Impairment of Long Lived Assets
Long-lived asset impairment	$ 0
Warrants
Warrant liability		$ 365
Stock-based compensation
Expected dividend yield	0.00%
Segment and Geographic Information
Number of operating segments \| segment	1

Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Restricted Cash
Restricted cash included in current assets	$ 119	$ 119
Restricted cash included in long-term assets	1,266	70
Corporate Headquarters Lease
Restricted Cash
Restricted cash included in long-term assets	1,300
Restricted by bank
Restricted Cash
Restricted cash, current and noncurrent	1,400	200
Stand-by letter of credit to landlord in connection with lease \| Corporate Headquarters Lease
Restricted Cash
Restricted cash included in current assets	$ 100	100
Restricted cash related to corporate credit card agreement
Restricted Cash
Restricted cash included in long-term assets		$ 100

Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property and Equipment, Net
Gross property and equipment	$ 8,738	$ 3,028
Less: accumulated depreciation and amortization	(2,077)	(1,546)
Property, Plant and Equipment, Net, Total	6,661	1,482
Depreciation expense	$ 900	600	$ 500
Lab equipment
Property and Equipment, Net
Estimated Useful Life (Years)	P5Y
Gross property and equipment	$ 2,712	2,000
Furniture and fixtures
Property and Equipment, Net
Estimated Useful Life (Years)	P4Y
Gross property and equipment	$ 632	326
Computer equipment
Property and Equipment, Net
Estimated Useful Life (Years)	P3Y
Gross property and equipment	$ 615	369
Leasehold improvements
Property and Equipment, Net
Gross property and equipment	$ 4,612	191
Software
Property and Equipment, Net
Estimated Useful Life (Years)	P3Y
Gross property and equipment	$ 167	$ 142

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accrued Expenses
Employee compensation	$ 2,731	$ 623
External research and development	1,471	2,034
Property and equipment costs	994
Professional fees and other	1,035	386
Consulting	165	216
Interest	34	150
Severance	6	330
Deferred offering costs		71
Total accrued expenses	$ 6,436	$ 3,810

Term Loan (Details) $ in Thousands	1 Months Ended					12 Months Ended
Term Loan (Details) $ in Thousands	Nov. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	May 31, 2013 USD ($) item	Dec. 31, 2015	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Term Loan
Proceeds from bank debt							$ 7,000	$ 3,000
Loan and Security Agreement
Term Loan
Maximum borrowing capacity					$ 5,000
Number of tranches \| item					3
Increase in maximum borrowing capacity	$ 5,000
Percentage fee payable on total loan advances at end of loan						4.00%
Percentage fee payable on outstanding principal in event of prepayment of loan advances, option 1						1.00%
Percentage fee payable on outstanding principal in event of prepayment of loan advances, option 2.						2.00%
Loan and Security Agreement \| 2013 Term Loan
Term Loan
Proceeds from bank debt		$ 2,000	$ 2,000	$ 1,000
Loan and Security Agreement \| 2013 Term Loan \| Prime Rate
Term Loan
Fixed rate based on percentage points added to base rate					2.00%
Loan and Security Agreement \| 2014 Term Loan
Term Loan
Proceeds from bank debt	$ 5,000

Term Loan - Principal Payments (Details) - Loan and Security Agreement $ in Thousands	Dec. 31, 2015 USD ($)
Term Loan
2016	$ 3,333
2017	2,583
2018	1,528
Total	$ 7,444

Warrants - Warrants (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
	May 05, 2015	Nov. 30, 2014	May 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Series A and Series B Warrants \| Maximum
Weighted-average assumptions
Interest expense				$ 0.1	$ 0.1	$ 0.1
Series A Warrants
Class Of Warrants Or Right [Line Items]
Exercise price of warrants issued (in dollars per share)	$ 5.50
Number of common shares that became exercisable from warrants	27,272
Weighted-average assumptions
Fair value of underlying instrument	$ 20.82		$ 1.00		$ 1.69
Expected volatility	91.58%		80.70%		89.98%
Expected term	8 years 1 month 6 days		10 years		8 years 4 months 24 days
Risk-free interest rate	2.06%		2.58%		2.15%
Series B Warrants
Class Of Warrants Or Right [Line Items]
Exercise price of warrants issued (in dollars per share)	$ 6.60
Number of common shares that became exercisable from warrants	15,151
Weighted-average assumptions
Fair value of underlying instrument	$ 20.82	$ 1.97			$ 1.97
Expected volatility	87.75%	87.18%			87.38%
Expected term	9 years 6 months	10 years			9 years 9 months 18 days
Risk-free interest rate	2.19%	2.36%			2.24%
Loan and Security Agreement \| Series A and Series B Warrants
Class Of Warrants Or Right [Line Items]
Warrants life				10 years
Loan and Security Agreement \| 2013 Term Loan \| Series A Warrants
Class Of Warrants Or Right [Line Items]
Number of shares that can be purchased by the warrants issued			150,000
Exercise price of warrants issued (in dollars per share)			$ 1.00
Loan and Security Agreement \| 2014 Term Loan \| Series B Warrants
Class Of Warrants Or Right [Line Items]
Number of shares that can be purchased by the warrants issued		83,333
Exercise price of warrants issued (in dollars per share)		$ 1.20

Warrants - Warrants Activity (Details) - USD ($) $ in Thousands			12 Months Ended
	May 13, 2015	May 05, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class Of Warrants Or Right [Line Items]
Fair value of warrants				$ 365
Fair Value Adjustment of Warrants			$ 445	$ 100	$ (4)
Reclassification of Warrant Liability to Additional Paid In Capital			810
Series A and Series B Warrants
Class Of Warrants Or Right [Line Items]
Fair value of warrants		$ 800
Reclassification of Warrant Liability to Additional Paid In Capital		$ 800
Other expense \| Series A and Series B Warrants
Class Of Warrants Or Right [Line Items]
Fair Value Adjustment of Warrants			$ 400
Loan and Security Agreement \| 2013 Term Loan \| Series A Warrants
Class Of Warrants Or Right [Line Items]
Stock issued upon exercise of warrants	21,281
Number of warrants cancelled	5,991
Loan and Security Agreement \| 2014 Term Loan \| Series B Warrants
Class Of Warrants Or Right [Line Items]
Stock issued upon exercise of warrants	11,157
Number of warrants cancelled	3,994

Convertible Preferred Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended					9 Months Ended	12 Months Ended
	May 31, 2015 item shares	Nov. 30, 2014 USD ($) $ / shares shares	Jan. 31, 2014 USD ($) $ / shares shares	Sep. 30, 2013 $ / shares shares	Jan. 31, 2013 $ / shares shares	Sep. 30, 2013 USD ($) $ / shares	Dec. 31, 2015 shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares	May 05, 2015 $ / shares
Preferred stock, shares authorized							5,000,000	5,000,000
Preferred stock, shares issued							0	0
Preferred stock, shares outstanding							0	0
IPO \| Minimum
Preferred stock, shares authorized	5,000,000
Number of series of preferred stock \| item	1
Common Stock
Conversion of preferred stock into common stock (in shares)							15,467,479
Common Stock \| IPO
Share price (in dollars per share) \| $ / shares										$ 18.00
Conversion of preferred stock into common stock (in shares)	15,467,479
Series A convertible preferred stock
Issuance of convertible preferred stock (in shares)				5,000,000	10,000,000				15,000,000
Share price (in dollars per share) \| $ / shares				$ 1.00	$ 1.00	$ 1.00			$ 1.00
Proceeds from issuance of convertible preferred stock, net of issuance costs \| $						$ 15,000			$ 14,987
Series B convertible preferred stock
Issuance of convertible preferred stock (in shares)			20,916,663					20,916,663
Share price (in dollars per share) \| $ / shares			$ 1.20					$ 1.20
Proceeds from issuance of convertible preferred stock, net of issuance costs \| $			$ 25,000					$ 24,985
Series C convertible preferred stock
Issuance of convertible preferred stock (in shares)		24,154,589						24,154,589
Share price (in dollars per share) \| $ / shares		$ 2.07						$ 2.07
Proceeds from issuance of convertible preferred stock, net of issuance costs \| $		$ 49,900						$ 49,868

Stock Awards - Unvested Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 01, 2016	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 08, 2015
Nonvested stock awards, Weighted-Average Grant Date Fair Value
Share-based compensation expense recorded				$ 5,180	$ 2,326	$ 351
Restricted Stock
Unvested restricted stock rollforward
Unvested at beginning of period (in shares)				425,279
Vested ( in shares)				(271,958)
Repurchased (in shares)				(22,826)
Unvested at end of period (in shares)				130,495
Nonvested stock awards, Weighted-Average Grant Date Fair Value
Unvested at beginning or period (in dollars per share)	$ 0.60			$ 0.46
Vested (in dollars per share)				$ 0.39
Repurchased (in dollars per share)				0.50
Unvested at end of period (in dollars per share)				$ 0.60	$ 0.46
Shares for which performance condition considered probable of achievement (in shares)					181,818
Total fair value of restricted stock that vested				$ 4,900	$ 1,500	$ 500
Non-employee performance-based restricted stock
Nonvested stock awards, Weighted-Average Grant Date Fair Value
Share-based compensation expense recorded		$ 1,400	$ 900
2015 Stock Option and Incentive Plan
Stock Awards
Number of shares available for grant				1,439,717			1,460,084
Increase in number of shares available for grant	1,087,842
Increase in number of shares available for grant (as a percent)				4.00%
Minimum percentage of fair value on grant date that the exercise price is required to be for employee shareholders for which restrictions apply				10.00%
Minimum percentage of fair value on grant date that the exercise price is required to be for employee shareholders for which restrictions apply				110.00%
Award vesting period				4 years
Cliff vesting period for new employees				1 year
Award expiration period				10 years

Stock Awards - Stock Options (Details) - Employee Stock Options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Stock option rollforward
Outstanding at beginning of period (in shares)	1,501,912
Granted (in shares)	716,573
Exercised ( in shares)	(289,237)
Cancelled ( in shares)	(126,446)
Outstanding at end of period (in shares)	1,802,802	1,501,912
Exercisable at end of period (in shares)	430,939
Vested and expected to vest at end of period (in shares)	1,757,456
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 2.02
Granted (in dollars per share)	11.96
Exercised ( in dollars per share)	1.95
Cancelled ( in dollars per share)	3.50
Outstanding at end of period (in dollars per share)	5.88	$ 2.02
Exercisable at end of period (in dollars per share)	3.50
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price	$ 5.86
Other disclosures
Remaining contractual life outstanding	8 years 9 months 4 days	9 years 5 months 1 day
Remaining contractual life, exercisable at end of period	8 years 6 months 22 days
Remaining contractual life, vested and expected to vest at end of period	8 years 9 months 4 days
Aggregate intrinsic value, outstanding at beginning of period	$ 7,704
Aggregate intrinsic value, outstanding at end of period	37,008	$ 7,704
Aggregate intrinsic value, exercisable at end of period	9,847
Aggregate intrinsic value, vested and expected to vest at end of period	$ 36,103
Forfeiture rate (as a percent)	2.50%

Stock Awards - Weighted Average Assumptions (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assumptions used in estimating the fair value of stock options granted
Expected dividend yield	0.00%
Employee Stock Options
Assumptions used in estimating the fair value of stock options granted
Risk-free interest rate	1.66%	1.92%	1.87%
Expected term (in years)	6 years	6 years 1 month 6 days	6 years 1 month 6 days
Expected stock price volatility	85.43%	92.99%	88.96%
Other disclosures
Weighted-average grant date fair value of options granted	$ 8.55	$ 3.46	$ 1.18
Total intrinsic value of options exercised	$ 6.8	$ 0.1	$ 0.0

Stock Awards - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock-based compensation
Allocated Share-based Compensation Expense	$ 5,180	$ 2,326	$ 351
Total unrecognized compensation cost related to non-vested stock option awards	$ 8,100
Weighted-average period over which unrecognized compensation cost will be recognized	2 years 4 months 10 days
Research and development
Stock-based compensation
Allocated Share-based Compensation Expense	$ 2,148	1,052	224
General and administrative
Stock-based compensation
Allocated Share-based Compensation Expense	$ 3,032	$ 1,274	$ 127

Stock Awards - Employee Stock Purchase Plan (Details) - 2015 Employee Stock Purchase Plan - shares	1 Months Ended
	May 31, 2015	Jan. 01, 2016
Stock Awards
Number of common shares reserved for future issuance of awards	243,347
Annual increase for common stock for issuance (as a percent)	1.00%
Increase of common shares reserved for future issuance		271,960

Net Loss per Share (Details) - shares	12 Months Ended
Net Loss per Share (Details) - shares	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	1,933,297	17,437,093	8,521,960
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted	18,235,614	1,420,518	1,015,000
Convertible preferred stock
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share		15,467,479	7,272,726
Warrant
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share		42,423	27,272
Stock options
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	1,802,802	1,501,912	385,250
Restricted Stock
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	130,495	425,279	836,712

Income Taxes - Reconciliation (Details)	12 Months Ended
Income Taxes - Reconciliation (Details)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	(3.30%)	(1.87%)	0.38%
Federal research and development credits	1.64%	1.30%	2.08%
State income tax, net of federal benefit	4.72%	4.93%	5.16%
Other	0.56%	0.70%	1.46%
Change in valuation allowance	(37.62%)	(39.06%)	(43.08%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 48,291	$ 30,603	$ 15,993
Research and development credit carryforwards	3,118	1,949	1,123
Deferred lease incentive	1,551
Accrued expenses and other	1,374	628	294
Deferred rent	331	54	109
Total gross deferred tax asset	54,665	33,234	17,519
Deferred tax liability	(1,702)	(85)	(139)
Debt discount	(39)	(74)	(41)
Valuation allowance	(52,924)	(33,075)	(17,339)
Net deferred tax asset	0	0	0
Change in valuation allowance	$ 19,800	$ 15,700	$ 9,000

Income Taxes - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Operating Loss and Tax Credit Carryforwards
Net NOLs related to stock compensation	$ 0.6
Federal orphan drug tax credits	0.3
Uncertain tax positions
Accrued interest related to uncertain tax positions	0.0	$ 0.0
Federal
Operating Loss and Tax Credit Carryforwards
Net operating loss carryforwards	123.2
State
Operating Loss and Tax Credit Carryforwards
Net operating loss carryforwards	121.3
Research and development tax credit \| Federal
Operating Loss and Tax Credit Carryforwards
Tax credit carryforwards	2.1
Research and development tax credit \| State
Operating Loss and Tax Credit Carryforwards
Tax credit carryforwards	$ 1.1

Commitments and Contingencies (Details) $ in Thousands	Feb. 12, 2015 USD ($) ft²	Dec. 31, 2015 USD ($)
Operating Leases
Total commitment under the operating lease		$ 17,350
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
Reimbursement of leasehold improvement costs included in current assets		$ 1,300
Security deposit included in restricted cash	$ 1,300

Commitments and Contingencies - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments and Contingencies Disclosure
Rent expense	$ 1,800	$ 800	$ 600
Future minimum lease payments
2016	2,326
2017	2,396
2018	2,468
2019	2,542
2020	2,618
Thereafter	5,000
Total minimum lease payments	$ 17,350

Collaborations (Details) $ in Thousands	1 Months Ended	3 Months Ended					12 Months Ended
Collaborations (Details) $ in Thousands	Mar. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) item
Collaboration Agreement
Collaboration revenue			$ 4,635	$ 3,426	$ 2,687	$ 652	$ 11,400
Unbilled accounts receivable for reimbursable research and development costs			3,414				$ 3,414
Alexion
Collaboration Agreement
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	$ 15,000
Payment receivable upon achievement of milestone							$ 250,000
Royalty term from first commercial sale, option 1							10 years
Royalty term from first commercial sale, option 2							15 years
Written termination notice period							90 days
Number of deliverables \| item							3
Collaboration revenue							$ 11,400
Reimbursable research and development costs							4,900
Unbilled accounts receivable for reimbursable research and development costs			3,200				3,200
Alexion \| Pre-clinical milestone for first licensed product
Collaboration Agreement
Payment receivable upon achievement of milestone		$ 750	$ 250		$ 1,750
Alexion \| Pre-clinical milestone for first licensed product \| Maximum
Collaboration Agreement
Payment receivable upon achievement of milestone							6,000
Alexion \| Development milestone for first licensed product \| Maximum
Collaboration Agreement
Payment receivable upon achievement of milestone							83,000
Alexion \| Development milestone for second licensed product \| Maximum
Collaboration Agreement
Payment receivable upon achievement of milestone							61,500
Alexion \| Commercial milestone
Collaboration Agreement
Payment receivable upon achievement of milestone							$ 51,000

Related-Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Third Rock Ventures LLC
Related-Party Transactions
Purchases from related party	$ 0.0	$ 0.4	$ 0.4

Defined Contribution Benefit Plan (Details) - USD ($) $ in Millions		12 Months Ended
	Sep. 01, 2015	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		$ 0.1

Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Data (unaudited)
Total revenue	$ 4,635	$ 3,426	$ 2,687	$ 652					$ 11,400
Total operating expenses	20,056	15,903	15,083	12,002	$ 14,293	$ 10,289	$ 8,199	$ 6,953	63,044	$ 39,734	$ 21,000
Total other expense, net	(154)	(165)	(584)	(222)	(175)	(214)	(88)	(74)	(1,125)	(551)	88
Net loss	(15,575)	(12,642)	(12,980)	(11,572)	(14,468)	(10,503)	(8,287)	(7,027)	(52,769)	(40,285)	(20,912)
Net loss applicable to common stockholders	$ (15,575)	$ (12,642)	$ (13,863)	$ (13,842)	$ (16,373)	$ (11,816)	$ (9,585)	$ (8,276)	$ (55,922)	$ (46,050)	$ (23,782)
Net loss per share applicable to common stockholders - basic and diluted	$ (0.58)	$ (0.47)	$ (0.81)	$ (8.23)	$ (10.44)	$ (8.13)	$ (6.99)	$ (6.44)	$ (3.07)	$ (32.41)	$ (23.43)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details