Blueprint Medicines (BPMC) 10-Q 9 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 02, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	Blueprint Medicines Corp
Entity Central Index Key	1,597,264
Document Type	10-Q
Document Period End Date	Sep. 30, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		27,100,791
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 179,780	$ 47,240
Restricted cash	119	119
Unbilled accounts receivable	2,470
Prepaid expenses and other current assets	6,218	915
Total current assets	188,587	48,274
Property and equipment, net	4,805	1,482
Other assets	560	99
Restricted cash	1,336	70
Total assets	195,288	49,925
Current liabilities:
Accounts payable	1,562	814
Accrued expenses	7,809	3,810
Deferred rent, net of current portion	16	138
Restricted stock liability	9	298
Current portion of deferred revenue	6,339
Current portion of lease incentive obligation	578
Current portion of term loan payable	2,978	1,704
Total current liabilities	19,291	6,764
Deferred rent, net of current portion	705
Restricted stock liability, net of current portion	1	29
Warrant liability		365
Deferred revenue, net of current portion	8,522
Lease incentive obligation, net of current portion	3,515
Term loan, net of current portion	4,893	$ 7,338
Other long term liabilities	$ 219
Commitments
Stockholders' deficit:
Preferred stock
Common stock	$ 27	$ 2
Additional paid-in capital	277,515	2,822
Accumulated deficit	(119,400)	(82,206)
Total stockholders' equity (deficit)	158,142	(79,382)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 195,288	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)

Balance (Parenthetical) - $ / shares	Sep. 30, 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 per share)	$ 0.001	$ 0.001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	27,095,711	2,218,652
Common Stock, Shares, Outstanding	26,895,589	1,626,738
Series A convertible preferred stock
Temporary Equity Disclosures
Convertible preferred stock, par value (in NIS 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 NIS 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 NIS 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

Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues
Collaboration revenue	$ 3,426		$ 6,765
Operating expenses:
Research and development	11,681	$ 8,368	32,157	$ 20,511
General and administrative	4,222	1,921	10,832	4,929
Total operating expenses	15,903	10,289	42,989	25,440
Other income (expense):
Other income (expense), net	6	(94)	(435)	(75)
Interest expense	(171)	(120)	(535)	(302)
Total other income (expense)	(165)	(214)	(970)	(377)
Net loss	(12,642)	(10,503)	(37,194)	(25,817)
Convertible preferred stock dividends		(1,313)	(3,153)	(3,860)
Net loss applicable to common stockholders	$ (12,642)	$ (11,816)	$ (40,347)	$ (29,677)
Net loss per share applicable to common stockholders - basic and diluted	$ (0.47)	$ (8.13)	$ (2.64)	$ (21.65)
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted (unaudited)	26,835,277	1,453,032	15,297,907	1,370,722

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (37,194)	$ (25,817)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	615	416
Noncash interest expense	84	65
Change in fair value of warrant liability	445	76
Stock-based compensation	3,985	1,026
Changes in assets and liabilities:
Unbilled accounts receivable	(2,470)
Prepaid expenses and other current assets	(1,044)	(931)
Other assets	(555)
Accounts payable	684	(19)
Accrued expenses	1,544	1,464
Deferred revenue	14,861
Deferred rent	415	(100)
Net cash used in operating activities	(18,630)	(23,820)
Investing activities
Purchases of property and equipment	(1,129)	(301)
Restricted cash	(1,266)
Net cash used in investing activities	(2,395)	(301)
Financing activities
Proceeds from term loan		2,000
Principal payments on loan payable	(1,250)	(500)
Proceeds from IPO, net of commissions and underwriting discounts	156,815
Payment of offering costs	(2,046)
Proceeds from issuance of common stock, net of repurchases	46	267
Net cash provided by financing activities	153,565	26,752
Net increase in cash and cash equivalents	132,540	2,631
Cash and cash equivalents at beginning of period	47,240	1,987
Cash and cash equivalents at end of period	179,780	4,618
Supplemental cash flow information
Cash paid for interest	347	79
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	$ 2,808	201
Series B convertible preferred stock
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 24,985

Nature of Business

Nature of Business	9 Months Ended
	Sep. 30, 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 genomically defined diseases and to craft drug candidates with therapeutic windows that provide significant and durable clinical response to patients. 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 Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2015
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The unaudited interim condensed 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). Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the financial statements as of and for the year ended December 31, 2014 and notes thereto, included in the Company’s final prospectus for the IPO filed with the SEC pursuant to Rule 424(b)(4) on April 30, 2015 (the Prospectus). The unaudited interim condensed financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of September 30, 2015 and the results of its operations for the three and nine months ended September 30, 2015 and 2014 and cash flows for the nine months ended September 30, 2015 and 2014. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2015 are not necessarily indicative of the results for the year ending December 31, 2015, or for any future period. 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, 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 were automatically converted into warrants exercisable for 42,423 shares of common stock. Additionally, the Company is now 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 nine months ended September 30, 2015 is expected to impact the year-over-year comparability of the Company’s net loss per share calculations over the next year. 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 contains 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, payment for the achievement of the first milestone under the agreement and payments for certain research and development services. The Company has not received any other milestone payments under the agreement or earned royalty revenue as a result of product sales. See Note 9 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 thi s 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. 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 September 30, 2015 and December 31, 2014 (in thousands): September 30, December 31, 2015 2014 Money market fund $ $ 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 September 30, 2015, are classified below based on the fair value hierarchy described above: Active Observable Unobservable September 30, Markets Inputs Inputs Description 2015 (Level 1) (Level 2) (Level 3) Money market funds, included in cash equivalents $ $ $ — $ — 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) Money market funds, included in cash equivalents $ $ $ — $ — Preferred stock warrants — — At September 30, 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 (Note 6). 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 three and nine months ended September 30, 2015 and 2014 (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2015 2014 2015 2014 Beginning balance $ — $ $ $ Change in fair value — Reclassification of fair value to additional paid-in capital — — — Ending balance $ — $ $ — $ 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, $2,062 of these costs were recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the IPO. As of December 31, 2014, the Company recorded $91 of deferred offering costs, included in other assets in the accompanying balance sheet, in contemplation of its IPO. There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Prospectus. 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 is expected to 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 has not yet determined the potential effects of the adoption of this standard on its consolidated financial position, results of operations or cash flows. 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 is currently assessing the potential impact of the adoption of ASU 2015-03 on its financial statements. 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 Company is in process of evaluating the new guidance and determining the expected effect on its financial statements.

Restricted Cash

Restricted Cash	9 Months Ended
	Sep. 30, 2015
Restricted Cash
Restricted Cash	3. Restricted Cash At September 30, 2015 and December 31, 2014, $1.5 million and $0.2 million, respectively, of the Company’s cash is restricted by a bank. As of September 30, 2015, $0.1 million of the restricted cash is 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. On February 12, 2015, the Company entered into a lease for approximately 38,500 rentable square feet of office and laboratory space in Cambridge, Massachusetts, which the Company gained control over on June 15, 2015 and occupancy commenced in October 2015. The lease ends on October 31, 2022. The lease agreement required the Company to pay a security deposit of $1.3 million, which is included in long-term assets on the Company’s balance sheet as of September 30, 2015. In addition, $0.1 million of restricted cash is included in long ‑term assets related to the Company’s corporate credit card agreement as of September 30, 2015 and December 31, 2014.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2015
Accrued Expenses
Accrued Expenses	4. Accrued Expenses Accrued expenses consist of the following (in thousands): September 30, December 31, 2015 2014 Leasehold improvement costs $ $ — External research and development Employee compensation Professional fees and other Consulting Interest Severance — Deferred offering costs — $ $

Term Loan

Term Loan	9 Months Ended
	Sep. 30, 2015
Term Loan
Term Loan	5. 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 is 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. Future minimum payments, which include principal and interest due under each of the 2013 Term Loan and the 2014 Term Loan, are $0.7 million, in the aggregate, for the remainder of 2015.

Warrants

Warrants	9 Months Ended
	Sep. 30, 2015
Warrants
Warrants	6. 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 $ $ $ $ Expected volatility % % % % Expected term (in years) Risk-free interest rate % % % % 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 nine months ended September 30, 2015 and 2014. 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 $ $ Expected volatility % % Expected term (in years) Risk-free interest rate % % 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 nine months ended September 30, 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.

Stock Awards

Stock Awards	9 Months Ended
	Sep. 30, 2015
Stock Awards
Stock Awards	7. Stock Awards 2015 Stock Option and Incentive Plan On April 8, 2015, the 2015 Stock Option and Incentive Plan (the Plan) was adopted by the board of directors and approved by the stockholders and became effective upon the completion of the IPO. 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 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 1,460,084 shares of common stock for the issuance of awards under the 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. This number is subject to adjustment in the event of a stock split, stock dividend or other change in our capitalization. At September 30, 2015, there were 1,379,432 shares available for future grant under the Plan. ISOs may not be granted at less than fair value on the date of the grant. Furthermore, 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 $ Granted — — Vested Repurchased Unvested at September 30, 2015 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 nine months ended September 30, 2014 management concluded that the milestones associated with 90,909 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 $0.1 million and $0.2 million of stock-based compensation expense for non ‑employee performance ‑based awards in the three and nine months end September 30, 2014, respectively. In the three months ended December 31, 2014, management concluded that the milestones associated with an additional 90,909 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. 2 million of stock-based compensation expense for non ‑employee performance ‑based awards in the nine months ended September 30, 2015 related to both milestone achievements. 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 $ $ Granted Exercised Canceled Outstanding at September 30, 2015 $ $ Exercisable at September 30, 2015 $ $ Vested and expected to vest at September 30, 2015(1) $ $ (1) Represents the number of vested options as of September 30, 2015, plus the number of unvested options expected to vest as of September 30, 2015 based on a forfeiture rate of 2.5% . (2) Intrinsic value represents the amount by which the fair market value as of September 30, 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: Three Months Ended Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 Risk-free interest rate % % % % Expected dividend yield — % — % — % — % Expected term (years) Expected stock price volatility % % % % The weighted ‑average grant date fair value of options granted in the nine months ended September 30, 2015 and 2014 was $8.22 and $3.23 , respectively. The total intrinsic value of options exercised in the three and nine months ended September 30, 2015 was $4.2 million and $4.7 million, respectively. The total intrinsic value of options exercised in the three and nine months ended September 30, 2014 was less than $0.1 million. Total stock ‑based compensation expense recognized for all stock ‑based compensation awards in the statements of operations is as follows (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2015 2014 2015 2014 Research and development $ $ $ $ General and administrative Total stock-based compensation expense $ $ $ $ At September 30, 2015, there was $8.3 million of total unrecognized compensation cost related to nonvested stock awards, which is expected to be recognized over a weighted ‑average period of 2.75 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 On April 8, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan. A total of 243,347 shares of common stock were initially authorized for issuance under this plan. The 2015 Employee Stock Purchase Plan became effective upon the completion of the IPO. As of September 30, 2015, the first offering under the 2015 Employee Stock Purchase Plan has not occurred.

Net Loss Per Share Applicble to

Net Loss Per Share Applicble to Common Stockholders	9 Months Ended
	Sep. 30, 2015
Net Loss Per Share Applicable to Common Stockholders
Net Loss Per Share Applicable to Common Stockholders	8. Net Loss per Share Applicable to Common Stockholders 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. September 30, 2015 2014 Convertible preferred stock — Warrants — Stock options Unvested restricted stock Total The weighted average number of common shares used in net loss per share applicable to common stockholders on a basic and diluted basis were 26,835,277 and 1,453,032 for the three months ended September 30, 2015 and 2014, respectively and 15,297,907 and 1,370,722 for the nine months ended September 30, 2015 and 2014, respectively.

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2015
Collaboration Agreement
Collaboration Agreement	9. Collaboration Agreement 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 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 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 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 in the agreement is non-substantive due to the c ertainty at the date the arrangement was entered into that the event will be achieved. Once the milestone is achieved, the Company will recognize revenues from the related milestone payment over the period of performance. In May 2015, the Company achieved the first pre-clinical milestone under the agreement and received $1.75 million payment from Alexion in the second quarter of 2015. 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. 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 three and nine months ended September 30, 2015, the Company recognized revenue under the Alexion agreement of $3.4 million and $6.8 million, respectively.

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2015
Related-Party Transactions
Related-Party Transactions	10. 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 $0.3 million for these services during the nine months ended September 30, 2014. The Company did not receive any consulting services from Third Rock Ventures during the nine months ended September 30, 2015.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	11. Commitments and Contingencies On February 12, 2015, the Company entered into a lease for approximately 38,500 rentable square feet of office and laboratory space in Cambridge, Massachusetts, which the Company gained control over on June 15, 2015, and occupancy commenced in October 2015. The lease ends on October 31, 2022. The Company has an option to extend the lease for five additional years. The lease has a total commitment of $17.8 million over the seven year term. The Company has agreed to pay an initial annual base rent of approximately $2,312,000 , which rises periodically until it reaches approximately $2,760,000 . The Company is recording rent expense on a straight-line basis through the end of the lease term. The Company has recorded deferred rent on the condensed consolidated balance sheet at September 30, 2015, accordingly. The lease provides the Company with an allowance for leasehold improvements of $4.3 million. The Company accounts for leasehold improvement incentives as a reduction to rent expense ratably over the lease term. The balance from the leasehold improvement incentives is included in lease incentive obligations on the balance sheets. The lease agreement required the Company to pay a security deposit of $1.3 million, which is recorded in restricted cash on the Company’s balance sheet.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2015
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The unaudited interim condensed 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). Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the financial statements as of and for the year ended December 31, 2014 and notes thereto, included in the Company’s final prospectus for the IPO filed with the SEC pursuant to Rule 424(b)(4) on April 30, 2015 (the Prospectus). The unaudited interim condensed financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of September 30, 2015 and the results of its operations for the three and nine months ended September 30, 2015 and 2014 and cash flows for the nine months ended September 30, 2015 and 2014. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2015 are not necessarily indicative of the results for the year ending December 31, 2015, or for any future period. 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, 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 were automatically converted into warrants exercisable for 42,423 shares of common stock. Additionally, the Company is now 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 nine months ended September 30, 2015 is expected to impact the year-over-year comparability of the Company’s net loss per share calculations over the next year.
Use of Estimates	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	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 contains 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, payment for the achievement of the first milestone under the agreement and payments for certain research and development services. The Company has not received any other milestone payments under the agreement or earned royalty revenue as a result of product sales. See Note 9 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 thi s 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.
Cash Equivalents	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 September 30, 2015 and December 31, 2014 (in thousands): September 30, December 31, 2015 2014 Money market fund $ $
Fair Value of Financial Instruments	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 September 30, 2015, are classified below based on the fair value hierarchy described above: Active Observable Unobservable September 30, Markets Inputs Inputs Description 2015 (Level 1) (Level 2) (Level 3) Money market funds, included in cash equivalents $ $ $ — $ — 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) Money market funds, included in cash equivalents $ $ $ — $ — Preferred stock warrants — — At September 30, 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 (Note 6). 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 three and nine months ended September 30, 2015 and 2014 (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2015 2014 2015 2014 Beginning balance $ — $ $ $ Change in fair value — Reclassification of fair value to additional paid-in capital — — — Ending balance $ — $ $ — $
Deferred Offering Costs	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, $2,062 of these costs were recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the IPO. As of December 31, 2014, the Company recorded $91 of deferred offering costs, included in other assets in the accompanying balance sheet, in contemplation of its IPO. There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Prospectus.
Recent Accounting Pronouncements	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 is expected to 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 has not yet determined the potential effects of the adoption of this standard on its consolidated financial position, results of operations or cash flows. 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 is currently assessing the potential impact of the adoption of ASU 2015-03 on its financial statements. 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 Company is in process of evaluating the new guidance and determining the expected effect on its financial statements.

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2015
Summary of Significant Accounting Policies
Schedule of cash equivalents	Cash equivalents consist of the following at September 30, 2015 and December 31, 2014 (in thousands): September 30, December 31, 2015 2014 Money market fund $ $
Schedule of financial instruments measured at fair value	Financial instruments measured at fair value as of September 30, 2015, are classified below based on the fair value hierarchy described above: Active Observable Unobservable September 30, Markets Inputs Inputs Description 2015 (Level 1) (Level 2) (Level 3) Money market funds, included in cash equivalents $ $ $ — $ — 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) Money market funds, included in cash equivalents $ $ $ — $ — Preferred stock warrants — —
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 three and nine months ended September 30, 2015 and 2014 (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2015 2014 2015 2014 Beginning balance $ — $ $ $ Change in fair value — Reclassification of fair value to additional paid-in capital — — — Ending balance $ — $ $ — $

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2015
Accrued Expenses
Schedule of accrued expenses	Accrued expenses consist of the following (in thousands): September 30, December 31, 2015 2014 Leasehold improvement costs $ $ — External research and development Employee compensation Professional fees and other Consulting Interest Severance — Deferred offering costs — $ $

Warrants (Tables)

Warrants (Tables)	9 Months Ended
	Sep. 30, 2015
Class Of Warrants Or Right [Line Items]
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 $ $ $ $ Expected volatility % % % % Expected term (in years) Risk-free interest rate % % % % Expected dividend yield — % — % — % — %
IPO
Class Of Warrants Or Right [Line Items]
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 $ $ Expected volatility % % Expected term (in years) Risk-free interest rate % % Expected dividend yield — % — %

Stock Awards (Tables)

Stock Awards (Tables)	9 Months Ended
	Sep. 30, 2015
Stock Awards
Summary of unvested restricted stock	Weighted-Average Grant Date Shares Fair Value Unvested at December 31, 2014 $ Granted — — Vested Repurchased Unvested at September 30, 2015
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 $ $ Granted Exercised Canceled Outstanding at September 30, 2015 $ $ Exercisable at September 30, 2015 $ $ Vested and expected to vest at September 30, 2015(1) $ $ (1) Represents the number of vested options as of September 30, 2015, plus the number of unvested options expected to vest as of September 30, 2015 based on a forfeiture rate of 2.5% . Intrinsic value represents the amount by which the fair market value as of September 30, 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	Three Months Ended Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 Risk-free interest rate % % % % Expected dividend yield — % — % — % — % Expected term (years) Expected stock price volatility % % % %
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): Three Months Ended Nine Months Ended September 30, September 30, 2015 2014 2015 2014 Research and development $ $ $ $ General and administrative Total stock-based compensation expense $ $ $ $

Net Loss Per Share Applicble 22

Net Loss Per Share Applicble to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2015
Net Loss Per Share Applicable to Common Stockholders
Schedule of common stock equivalents excluded from calculation of diluted net loss per share applicable to common stockholders	September 30, 2015 2014 Convertible preferred stock — Warrants — Stock options Unvested restricted stock Total

Nature of Business (Details)

Nature of Business (Details) - USD ($) $ / shares in Units, $ in Thousands	May. 05, 2015	Sep. 30, 2015
Initial Public Offering
Proceeds from issuance of IPO shares		$ 156,815
IPO | Common Stock
Initial Public Offering
Stock sold (in shares)	9,367,708
Share price (in dollars per share)	$ 18
Proceeds from issuance of IPO shares	$ 154,800
Shares issued to underwriters in IPO	1,221,874

Summary of Significant Accoun24

Summary of Significant Accounting Policies - Stock Split nad Conversion of Stock (Details)	May. 05, 2015shares	Apr. 10, 2015	Sep. 30, 2015shares	Dec. 31, 2014shares
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 Accoun25

Summary of Significant Accounting Policies - Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Money market fund
Cash Equivalents
Cash equivalents	$ 179,780	$ 47,240

Summary of Significant Accoun26

Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair value liabilities measured on recurring basis unobservable input reconciliation
Change in fair value of warrant liability		$ 445	$ 76
Reclassification of Warrant Liability to Additional Paid In Capital		(810)
Recurring
Fair value liabilities measured on recurring basis unobservable input reconciliation
Beginning balance		365
Preferred stock warrants		(365)		$ (365)
Recurring | Money market fund
Fair value liabilities measured on recurring basis unobservable input reconciliation
Cash equivalents		179,780		47,240
Recurring | Active Markets (Level 1) | Money market fund
Fair value liabilities measured on recurring basis unobservable input reconciliation
Cash equivalents		179,780		47,240
Recurring | Unobservable Inputs (Level 3)
Fair value liabilities measured on recurring basis unobservable input reconciliation
Beginning balance	$ 100	365	119
Change in fair value of warrant liability	95	445	76
Reclassification of Warrant Liability to Additional Paid In Capital		(810)
Ending balance	195		195
Preferred stock warrants	$ (100)	$ (365)	$ (119)	$ (365)

Summary of Significant Accoun27

Summary of Significant Accounting Policies - Deferred Offering Costs (Details) - USD ($) $ in Thousands	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	$ 2,062
Deferred Offering Costs		$ 91

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Restricted Cash
Restricted cash included in current assets	$ 119	$ 119
Restricted cash included in long-term assets	1,336	70
Corporate Headquarters Lease
Restricted Cash
Security deposit	1,300
Restricted by bank
Restricted Cash
Restricted cash, current and noncurrent	1,500	200
Stand-by letter of credit to landlord in connection with lease | Corporate Headquarters Lease
Restricted Cash
Restricted cash included in current assets	100
Restricted cash related to corporate credit card agreement
Restricted Cash
Restricted cash included in long-term assets	$ 100	$ 100

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accrued Expenses
Leasehold improvement costs	$ 2,745
External research and pre-clinical development	2,177	$ 2,034
Employee compensation	1,759	623
Professional fees and other	902	386
Consulting	191	216
Interest	35	150
Severance		330
Deferred offering costs		71
Total accrued expenses	$ 7,809	$ 3,810

Term Loan (Details)

Term Loan (Details) - USD ($) $ in Thousands	1 Months Ended					9 Months Ended
	Nov. 30, 2014	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2013	May. 31, 2013	Sep. 30, 2015	Sep. 30, 2014
Term Loan
Proceeds from bank debt							$ 2,000
Loan and Security Agreement
Term Loan
Maximum borrowing capacity					$ 5,000
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%
Future minimum payments for remainder of 2015						$ 700
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

Warrants - Warrants (Details)

Warrants - Warrants (Details) - USD ($) $ / shares in Units, $ in Millions	May. 05, 2015	Nov. 30, 2014	May. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Series A and Series B Warrants | Maximum
Weighted-average assumptions
Interest expense				$ 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			$ 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
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 (D

Warrants - Warrants Activity (Details) - USD ($) $ in Thousands	May. 13, 2015	May. 05, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Class Of Warrants Or Right [Line Items]
Fair value of warrants					$ 365
Fair Value Adjustment of Warrants			$ 445	$ 76
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

Stock Awards - Unvested Restric

Stock Awards - Unvested Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Apr. 08, 2015
Nonvested stock awards, Weighted-Average Grant Date Fair Value
Share-based compensation expense recorded	$ 971	$ 567	$ 3,985	$ 1,026
Restricted Stock
Unvested restricted stock rollforward
Unvested at beginning of period (in shares)			425,279
Vested ( in shares)			(203,056)
Repurchased (in shares)			(22,101)
Unvested at end of period (in shares)	200,122		200,122
Nonvested stock awards, Weighted-Average Grant Date Fair Value
Unvested at beginning or period (in dollars per share)			$ 0.46
Vested (in dollars per share)			$ 0.32
Repurchased (in dollars per share)			0.47
Unvested at end of period (in dollars per share)	$ 0.55		$ 0.55		$ 0.46
Shares for which performance condition considered probable of achievement (in shares)				90,909	90,909
Non-employee performance-based restricted stock
Nonvested stock awards, Weighted-Average Grant Date Fair Value
Share-based compensation expense recorded		$ 100	$ 1,000	$ 200
2015 Stock Option and Incentive Plan
Stock Awards
Number of shares available for grant	1,379,432		1,379,432			1,460,084
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 (D

Stock Awards - Stock Options (Details) - Employee Stock Options $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015USD ($)$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares
Stock option rollforward
Outstanding at beginning of period (in shares) | shares	1,501,912
Granted (in shares) | shares	681,073
Exercised ( in shares) | shares	(188,170)
Cancelled ( in shares) | shares	(124,400)
Outstanding at end of period (in shares) | shares	1,870,415	1,501,912
Exercisable at end of period (in shares) | shares	418,243
Vested and expected to vest at end of period (in shares) | shares	1,777,920
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 2.02
Granted (in dollars per share)	11.46
Exercised ( in dollars per share)	1.86
Cancelled ( in dollars per share)	3.49
Outstanding at end of period (in dollars per share)	5.38	$ 2.02
Exercisable at end of period (in dollars per share)	2.91
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price	$ 5.37
Other disclosures
Remaining contractual life outstanding	8 years 8 months 12 days	9 years 5 months 1 day
Remaining contractual life, exercisable at end of period	7 years 6 months
Remaining contractual life, vested and expected to vest at end of period	8 years 8 months 5 days
Aggregate intrinsic value, outstanding at beginning of period | $	$ 7,704
Aggregate intrinsic value, outstanding at end of period | $	30,353	$ 7,704
Aggregate intrinsic value, exercisable at end of period | $	7,719
Aggregate intrinsic value, vested and expected to vest at end of period | $	$ 28,860
Forfeiture rate (as a percent)	0.025

Stock Awards - Weighted Average

Stock Awards - Weighted Average Assumptions (Details) - Employee Stock Options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Assumptions used in estimating the fair value of stock options granted
Minimum risk-free interest rate	1.71%		1.66%
Risk-free interest rate		1.94%		1.93%
Expected term (in years)	6 years	6 years	6 years	6 years
Expected stock price volatility	81.85%	90.95%	85.59%	91.01%
Other disclosures
Weighted-average grant date fair value of options granted			$ 8.22	$ 3.23
Total intrinsic value of options exercised	$ 4.2	$ 0.1	$ 4.7	$ 0.1

Stock Awards - Stock-based Comp

Stock Awards - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Stock-based compensation
Allocated Share-based Compensation Expense	$ 971	$ 567	$ 3,985	$ 1,026
Total unrecognized compensation cost related to non-vested stock option awards	8,300		$ 8,300
Weighted-average period over which unrecognized compensation cost will be recognized			2 years 9 months
Research and development
Stock-based compensation
Allocated Share-based Compensation Expense	329	226	$ 1,618	481
General and administrative
Stock-based compensation
Allocated Share-based Compensation Expense	$ 642	$ 341	$ 2,367	$ 545

Stock Awards - Employee Stock P

Stock Awards - Employee Stock Purchase Plan (Details)	Apr. 08, 2015shares
2015 Employee Stock Purchase Plan
Stock Awards
Number of common shares reserved for future issuance of awards	243,347

Net Loss Per Share Applicble 38

Net Loss Per Share Applicble to Common Stockholders (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share			2,070,537	13,149,341
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted (unaudited)	26,835,277	1,453,032	15,297,907	1,370,722
Convertible preferred stock
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share				11,075,750
Warrant
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share				27,272
Stock options
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share			1,870,415	1,492,029
Restricted Stock
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share			200,122	554,290

Collaboration Agreement (Detail

Collaboration Agreement (Details) $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	May. 31, 2015USD ($)	Mar. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2015USD ($)item
Collaboration Agreement
Collaboration revenue			$ 3,426	$ 6,765
Alexion
Collaboration Agreement
Collaborative partner's funding responsbility 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			$ 3,400	$ 6,800
Alexion | Pre-clinical milestone for first licensed product
Collaboration Agreement
Payment receivable upon achievement of milestone	$ 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 (Det

Related-Party Transactions (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Third Rock Ventures LLC
Related-Party Transactions
Purchases from related party	$ 0	$ 300,000

Commitments and Contingencies (

Commitments and Contingencies (Details) - Office and laboratory space in Cambridge, Massachusetts	Feb. 12, 2015USD ($)ft²
Operating Leased Assets [Line Items]
Area leased (in square feet) | ft²	38,500
Extension period of lease term	5 years
Total commitment under the operating lease	$ 17,800,000
Lease term	7 years
Base annual rent, initial	$ 2,312,000
Base annual rent, maximum	2,760,000
Allowance for leasehold improvements	4,300,000
Security deposit restricted cash	$ 1,300,000