Dimension Therapeutics (DMTX) 10-K 9 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Mar. 02, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	DMTX
Entity Registrant Name	Dimension Therapeutics, Inc.
Entity Central Index Key	1,592,288
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		25,043,506
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 103.7

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 30,234	$ 127,047
Marketable securities	47,715	0
Accounts receivable	1,885	143
Prepaid expenses and other current assets	5,484	2,740
Total current assets	85,318	129,930
Property and equipment, net	8,402	3,339
Deferred offering costs	145	0
Total assets	93,865	133,269
Current liabilities:
Accounts payable	2,368	1,555
Accrued expenses and other current liabilities	7,247	3,715
Amounts due to related parties	0	522
Deferred revenue	8,663	6,835
Notes payable	2,361	574
Total current liabilities	20,639	13,201
Deferred revenue, net of current portion	8,663	13,670
Notes payable, net of discount and current portion	4,169	759
Other liabilities	453	56
Total liabilities	33,924	27,686
Commitments and contingencies (Note 14)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized at December 31, 2016 and 2015; zero shares issued or outstanding at December 31, 2016 and 2015.	0	0
Common stock, $0.0001 par value; 150,000,000 shares authorized as of December 31, 2016 and 2015; 25,043,506 and 25,008,227 shares issued and outstanding as of December 31, 2016 and 2015, respectively.	2	2
Additional paid-in capital	160,185	156,775
Accumulated deficit	(100,195)	(51,194)
Accumulated other comprehensive loss	(51)	0
Total stockholders’ equity	59,941	105,583
Total liabilities and stockholders’ equity	$ 93,865	$ 133,269

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	25,043,506	25,008,227
Common stock, shares outstanding	25,043,506	25,008,227

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 11,471	$ 7,750	$ 2,750
Operating expenses:
Research and development	47,741	33,992	12,974
General and administrative	12,780	8,723	2,727
Total operating expenses	60,521	42,715	15,701
Loss from operations	(49,050)	(34,965)	(12,951)
Interest income (expense), net	49	(91)	(17)
Net loss	(49,001)	(35,056)	(12,968)
Accretion of convertible preferred stock to redemption value	0	(23)	(71)
Net loss attributable to common stockholders	$ (49,001)	$ (35,079)	$ (13,039)
Net loss per share attributable to common stockholders — basic and diluted	$ (1.97)	$ (4.41)	$ (3.61)
Weighted average common shares outstanding — basic and diluted	24,915,887	7,949,670	3,610,592
Comprehensive loss:
Net loss	$ (49,001)	$ (35,056)	$ (12,968)
Other comprehensive loss
Unrealized loss on marketable securities	(51)
Total other comprehensive gain (loss)	(51)
Total comprehensive loss	$ (49,052)	$ (35,056)	$ (12,968)

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A and B Convertible Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member]Series B Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2013		$ 4,707
Beginning balance at Dec. 31, 2013	$ (1,285)					$ 1,885	$ (3,170)
Beginning balance, shares at Dec. 31, 2013		5,000,000
Beginning balance, shares at Dec. 31, 2013					3,715,245
Issuance of convertible preferred stock, net of issuance costs, value			$ 4,875
Issuance of convertible preferred stock, net of issuance costs, shares			5,000,000
Issuance of common stock warrant in connection with notes payable	5					5
Vesting of restricted common stock issued for consulting services, value	42					42
Issuance of restricted common stock, shares					341,573
Stock-based compensation expense	86					86
Temporary equity, Accretion of convertible preferred stock to redemption value		$ 71
Accretion of convertible preferred stock to redemption value	(71)					(71)
Net loss	(12,968)						(12,968)
Ending balance at Dec. 31, 2014		$ 9,653
Ending balance at Dec. 31, 2014	(14,191)					1,947	(16,138)
Ending balance, shares at Dec. 31, 2014		10,000,000
Ending balance, shares at Dec. 31, 2014					4,056,818
Issuance of convertible preferred stock, net of issuance costs, value			$ 14,480	$ 64,739
Issuance of convertible preferred stock, net of issuance costs, shares			14,500,000	20,183,824
Issuance of restricted common stock, shares					35,788
Exercise of common stock options, value	5					5
Exercise of common stock options, shares					12,572
Exercise of common stock warrants, shares					11,306
Stock-based compensation expense	1,333					1,333
Temporary equity, Accretion of convertible preferred stock to redemption value		$ 23
Accretion of convertible preferred stock to redemption value	(23)					(23)
Conversion of redeemable convertible preferred stock to common stock, value	88,895	$ (88,895)			$ 1	88,894
Conversion of redeemable convertible preferred stock to common stock, shares		(44,683,824)			15,286,968
Initial public offering of common stock, net of commissions, underwriting discounts and offering costs	64,620				$ 1	64,619
Initial public offering of common stock, net of commissions, underwriting discounts and offering costs, shares					5,604,775
Net loss	(35,056)						(35,056)
Ending balance at Dec. 31, 2015	$ 105,583				$ 2	156,775	(51,194)
Ending balance, shares at Dec. 31, 2015	25,008,227				25,008,227
Exercise of common stock options, value	$ 43					43
Exercise of common stock options, shares	35,279				35,279
Stock-based compensation expense	$ 3,367					3,367
Other comprehensive loss	(51)							$ (51)
Net loss	(49,001)						(49,001)
Ending balance at Dec. 31, 2016	$ 59,941				$ 2	$ 160,185	$ (100,195)	$ (51)
Ending balance, shares at Dec. 31, 2016	25,043,506				25,008,227

Consolidated Statements of Con6

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Series A Convertible Preferred Stock [Member]
Issuance of convertible preferred stock, issuance costs	$ 20	$ 125
Series B Convertible Preferred Stock [Member]
Issuance of convertible preferred stock, issuance costs	$ 261

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (49,001)	$ (35,056)	$ (12,968)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Consulting services paid in common stock	0	0	42
Depreciation and amortization expense	1,652	716	164
Stock-based compensation expense	3,367	1,333	86
Non-cash interest expense	115	9	3
Premium on marketable securities	(207)	0	0
Amortization of premium on marketable securities	37	0	0
Changes in operating assets and liabilities:
Accounts receivable	(1,742)	1,831	(1,973)
Prepaid expenses and other current assets	(2,744)	(2,336)	(375)
Deferred offering costs	(145)	0	0
Accounts payable	814	1,148	785
Accrued expenses and other current liabilities and amounts due to related parties	2,992	2,407	354
Deferred revenue	(3,179)	(2,137)	22,642
Other liabilities	397	56	0
Net cash (used in) provided by operating activities	(47,644)	(32,029)	8,760
Cash flows from investing activities:
Purchases of marketable securities	(54,196)	0	0
Sales and maturities of marketable securities	6,600	0	0
Purchases of property and equipment	(6,716)	(2,543)	(1,667)
Lease deposits	0	62	(88)
Net cash used in investing activities	(54,312)	(2,481)	(1,755)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	79,219	4,875
Proceeds from exercise of common stock options	43	5	0
Proceeds from issuance of notes payable and common stock warrant, net of issuance costs	5,779	194	1,526
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	0	67,762	0
Payments of initial public offering costs	0	(3,142)	0
Repayment of notes payable	(679)	(394)	0
Net cash provided by financing activities	5,143	143,644	6,401
Net increase (decrease) in cash and cash equivalents	(96,813)	109,134	13,406
Cash and cash equivalents at beginning of period	127,047	17,913	4,507
Cash and cash equivalents at end of period	30,234	127,047	17,913
Supplemental disclosure of cash flow information:
Interest paid	136	82	14
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value	0	23	71
Issuance of common stock warrant in connection with notes payable	0	0	5
Purchases of property and equipment included in accounts payable and accrued expenses	8	9	277
Conversion of preferred stock to common stock upon initial public offering	$ 0	$ 88,895	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Dimension Therapeutics, Inc. (the “Company”) was incorporated in Delaware on June 20, 2013. The Company is the leader in discovering and developing new therapeutic products for people living with devastating rare and metabolic diseases associated with the liver, based on the most advanced mammalian adeno-associated virus (AAV) gene delivery technology. The liver is a vital organ that plays an important role in human metabolism and key physiologic functions, and is the target organ for our initial programs: OTC deficiency, GSDIa, citrullinemia type I, phenylketonuria, Wilson disease, hemophilia B, and hemophilia A. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. In accordance with ASC 205-40, Going Concern, the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. As of December 31, 2016 the Company had an accumulated deficit of $100,195. During the year ended December 31, 2016, the Company incurred a loss of $49,001 and used $47,644 of cash in operations. The Company expects to continue to generate operating losses in the foreseeable future. The Company expects that its cash, cash equivalents and marketable securities of $77,949 at December 31, 2016, together with the receipt of contingent payments expected to be received in connection with our collaboration with Bayer, including reimbursements and $15 million in milestones, and cost management initiatives, will be sufficient to fund its operating expenses and capital expenditure requirements for at least the next 12 months from issuance of the financial statements. If the Company does not obtain the milestone payments, the Company would be forced to delay, reduce or eliminate certain research and development programs, reduce or eliminate discretionary operating expenses, delay product portfolio expansion, which could adversely affect its business prospects. In addition, the Company will seek additional funding through public or private financings, debt financing, collaboration agreements or government grants. The inability to obtain funding, as and when needed, would have a negative impact on the Company’s financial condition and ability to pursue its business strategies. The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements reflect the operations of Dimension Therapeutics, Inc. and Dimension Securities Corporation, our wholly-owned subsidiary. All intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, revenue recognition, the accrual for research and development expenses and the valuation of common stock. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. Concentrations of Credit Risk, Significant Customers and Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities and accounts receivable. The Company has all cash, cash equivalents and marketable securities balances at one accredited financial institution in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. As of December 31, 2016 and 2015 the Company's accounts receivable balances resulted from its collaboration agreement with Bayer (see Note 8), and the Company believes Bayer to be creditworthy. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of an equity financing, these costs are recorded in stockholders' equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. The Company has recorded deferred offering costs of $145 as of December 31, 2016 related to its At-the-Market (ATM) agreement entered into in connection with its Form S-3 Registration Statement prospectus dated November 10, 2016. Should shares under the AMT no longer be considered probable, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive loss. The Company did not record any deferred offering costs as of December 31, 2015. Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents, which consist of money market funds and U.S. government securities, are stated at fair value. Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. Property and Equipment Property and equipment are recorded at cost and depreciated over their estimated useful lives using the straight-line method over the estimated useful life of each asset. Computer equipment is depreciated over five years. Laboratory equipment is depreciated over five years. Furniture and fixtures are depreciated over seven years. Leasehold improvements are amortized over the shorter of the lease term or the five-year estimated useful life of the asset. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Repairs and maintenance costs are expensed as incurred. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets. Fair Value Measurements Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 — Quoted prices in active markets for identical assets or liabilities. • Level 2 — Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Debt Issuance Costs The Company has adopted the amendments to the Financial Accounting Standards Board ("FASB") Accounting Standards Codification made in Accounting Standards Update ("ASU") 2015-03, Interest — Imputation of Interest (Subtopic 835-30) ("ASU 2015-03") simplifying the presentation of debt issuance costs. According to ASU 2015-03, the Company presents debt issuance costs related to a recognized debt liability as a direct reduction from the carrying amount of that debt liability. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. All of the Company's tangible assets are held in the United States. To date, all of the Company's revenue has been generated in the United States. Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition. Accordingly, the Company recognizes revenue for each unit of accounting 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. Collaborative Research and License Agreements The terms of these agreements contain multiple deliverables, which may include licenses and research and development activities. The terms of these agreements may also include nonrefundable upfront license fees, payments for research and development activities, reimbursement of certain third-party costs, payments based upon the achievement of specified milestones, and royalty payments based on product sales derived from the collaboration. The Company evaluates its collaborative agreements for proper classification in its statements of operations and comprehensive loss based on the nature of the underlying activity. The Company generally reflects as revenue amounts due under its collaborative agreements related to reimbursement of development activities as the Company is generally the principal under the arrangement. The Company evaluates multiple-element arrangements based on the guidance in ASC Topic 605-25, Revenue Recognition — Multiple-Element Arrangements ("ASC 605-25"). Pursuant to the guidance in ASC 605-25, the Company evaluates multiple-element arrangements to determine (i) the deliverables included in the arrangement and (ii) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit. When the Company determines that an arrangement should be accounted for as a single unit of accounting, the Company must determine the period over which the performance obligations will be performed and revenue will be recognized. This evaluation requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that (i) the delivered item has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return with respect to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company's control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, 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 any other deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item, and whether there are other vendors that can provide the undelivered items. The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the selling price of a unit of accounting within each arrangement 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 as 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. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the contractual or estimated performance period for the undelivered items, which is typically the term of the Company's research and development obligations. If there is no discernible pattern of performance 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. Conversely, if the pattern of performance over 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. 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. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations. 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: (i) the consideration is commensurate with either the Company's performance to achieve the milestone or the enhancement of the value of the delivered item as a result of a specific outcome resulting from the Company's performance to achieve the milestone, (ii) the consideration relates solely to past performance, and (iii) 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 particular milestone and the level of effort and investment required to achieve the particular 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. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up being recognized for the elapsed portion of the period of performance, assuming all other revenue recognition criteria are met. To date, the Company has not recorded any substantive milestones because no milestones that meet the required criteria listed above have been identified. Payments for achievement of non-substantive milestones are deferred and recognized as revenue over the estimated period of performance applicable to the collaborative arrangement. As these milestones are achieved, the Company will recognize as revenue a portion of the milestone payment that is equal to the percentage of the performance period completed when the milestone is achieved, multiplied by the amount of the milestone payment, upon achievement of such milestone. The Company will recognize the remaining portion of the milestone payment over the remaining performance period under either the proportional performance method or on a straight-line basis. Amounts received prior to satisfying the revenue recognition criteria listed above are recorded as deferred revenue in the accompanying balance sheets. Although the Company follows detailed guidelines in measuring revenue, certain judgments affect the application of the Company's revenue policy. For example, in connection with its existing collaboration agreement, the Company has recorded on its balance sheet short-term and long-term deferred revenue based on its best estimate of when such revenue will be recognized. Short-term deferred revenue consists of amounts that are expected to be recognized as revenue in the next 12 months. Amounts that the Company expects will not be recognized within the next 12 months are classified as long-term deferred revenue. However, this estimate is based on the Company's current research plan and, if its research plan should change in the future, the Company may recognize a different amount of deferred revenue over the following 12-month period. The estimate of deferred revenue also reflects management's estimate of the periods of the Company's involvement in its collaboration. The Company's performance obligations under this collaboration consist of participation on steering committees and performance of other research and development services. In certain instances, the timing of satisfying these obligations can be difficult to estimate. Accordingly, the Company's estimates may change in the future. Such changes to estimates would result in a change in prospective revenue recognition amounts. If these estimates and judgments change over the course of any of the Company's collaborative agreements, it may affect the timing and amount of revenue that the Company will recognize and record in future periods. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third party license fees, and external costs of outside vendors engaged to conduct preclinical and clinical development activities and trials. Research and development expenses include the Company's costs of performing services in connection with its collaboration agreement with Bayer. Nonrefundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company's estimates. The Company's historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards with only service based vesting conditions and records the expense for these awards using the straight-line method. For stock-based awards granted to consultants and non-employees, compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company's common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its statement of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified. The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company's estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Prior to the Company’s initial public offering in October 2015, the Company was a private company and lacked company-specific historical and implied volatility information. Therefore, the Company estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company's stock options has been determined utilizing the "simplified" method for awards that qualify as "plain-vanilla" options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders' equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2016, the Company’s only element of other comprehensive loss was unrealized loss on marketable securities. For the years ended December 31, 2105 and 2014, there were no differences between net loss and comprehensive loss. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company's tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options, warrants and unvested restricted stock. Net loss per share attributable to common stockholders is calculated using the two-class method, which is an earnings allocation formula that determines net loss per share for the holders of the Company's common shares and participating securities. The Company's convertible preferred stock contains participation rights in any dividend paid by the Company and are deemed to be participating securities. Net loss attributable to common stockholders and participating preferred shares are allocated to each share on an as-converted basis as if all of the earnings for the period had been distributed. The participating securities do not include a contractual obligation to share in losses of the Company and are not included in the calculation of net loss per share in the periods that have a net loss. Diluted net loss per share is computed using the more dilutive of (a) the two-class method or (b) the if-converted method. The Company allocates earnings first to preferred stockholders based on dividend rights, and then to common and preferred stockholders based on ownership interests. The weighted average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options, warrants, unvested restricted common stock and convertible preferred stock. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since dilutive common shares are not assumed to have been issued if their effect is antidilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair Value Measurements and Marketable Securities Fair Value Measurements The Company’s cash equivalents are generally classified within Level 1 of the fair value hierarchy. The Company’s investments in marketable securities are classified within Level 2 of the fair value hierarchy. The fair values of the Company’s marketable securities are generally based on prices obtained from independent pricing sources. Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are generally reflected within Level 2, as they are primarily based on observable pricing for similar assets and/or other market observable inputs. Typical inputs used by these pricing services include, but are not limited to, reported trades, benchmark yields, issuer spreads, bids, offers and/or estimates cash flow, prepayment spreads and default rates. The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2016 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 22,988 $ — $ — $ 22,988 U.S. government securities — 7,246 — 7,246 Total cash equivalents 22,988 7,246 — 30,234 Marketable securities: Commercial paper — 15,814 — 15,814 Corporate Bonds — 13,671 — 13,671 U.S. government securities — 18,230 — 18,230 Total marketable securities — 47,715 — 47,715 Total cash equivalents and marketable securities $ 22,988 $ 54,961 $ — $ 77,949 Fair Value Measurements as of December 31, 2015 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 127,047 $ — $ — $ 127,047 Total cash equivalents $ 127,047 $ — $ — $ 127,047 During the year ended December 31, 2016 and 2015, there were no transfers between Level 1 and Level 2. The carrying amounts reflected in the consolidated balance sheets for accounts payable and accrued expenses approximate fair value due to their short-term maturities. The carrying value of the Company’s outstanding notes payable approximates fair value (a Level 2 fair value measurement), reflecting interest rates currently available to the Company. Marketable Securities The following tables summarize the Company’s marketable securities as of December 31, 2016: Fair Value Measurements as of December 31, 2016 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 15,814 $ — $ — $ 15,814 Corporate bonds (due within 1 year) 9,182 — (10 ) 9,172 U.S. government securities (due within 1 year) 7,002 — — 7,002 Corporate bonds (due after 1 year through 2 years) 4,512 — (12 ) 4,500 U.S. government securities (due after 1 year through 2 years) 11,256 — (29 ) 11,227 $ 47,766 $ — $ (51 ) $ 47,715

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Prepaid Expenses and Other Current Assets	4. Prepaid expenses and other current assets: Prepaid expenses and other current assets consisted of the following: December 31, 2016 2015 Prepaid clinical, manufacturing, and scientific costs $ 4,406 $ 1,824 Prepaid insurance 637 587 Other 441 329 $ 5,484 $ 2,740

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Property and Equipment, net	5. Property and Equipment, net Property and Equipment, net consisted of the following: December 31, Useful Life 2016 2015 Laboratory equipment 5 years $ 8,114 $ 3,663 Computer equipment 5 years 251 165 Furniture and fixtures 7 years 310 193 Leasehold improvements * 2,259 198 10,934 4,219 Less: Accumulated depreciation and amortization (2,532 ) (880 ) $ 8,402 $ 3,339 * Shorter of asset life or lease term Depreciation and amortization expense was $1,652, $716 and $164 for the years ended December 31, 2016, 2015 and 2014, respectively. Substantially all of the Company’s laboratory equipment server as collateral for the Company’s standby letter of credit pursuant to the agreement for the Company’s leased facilities (see Note 14).

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: December 31, 2016 2015 Payroll and payroll-related costs $ 2,234 $ 2,000 External research and development services 4,409 1,389 Professional fees 433 254 Other 171 72 $ 7,247 $ 3,715

Notes Payable

Notes Payable	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Notes Payable	7. Notes Payable In August, 2014, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”), which provided for advances under an equipment line of up to $1,750. As of December 31, 2015, the Company has borrowed the full amount of $1,750 under the loan and security agreement and incurred a total of $35 of related issuance costs. Under the loan and security agreement, for each advance under the equipment line, the Company is obligated to make six monthly, interest-only payments. Thereafter, the Company is obligated to pay 36 monthly installments of interest and principal. Outstanding principal under the loan and security agreement accrue interest at an annual rate of 5.5%, which was determined on the date of each drawdown based on an annual rate of 2.25% plus the greater of (1) 3.25% or (2) the rate of interest per annum from time to time published in the money rates section of The Wall Street Journal as the prime rate then in effect. Borrowings under the loan and security agreement are secured by substantially all of our assets, except for our intellectual property. In connection with entering into the loan and security agreement, in August 2014, the Company granted to the lender a warrant to purchase 11,973 shares of our common stock at an exercise price of $0.56 per share. This warrant was exercised at the option of the lender by the option of a cashless exercise in December 2015 resulting in the issuance of 11,306 of our common shares. Because the warrant is indexed to the Company's own stock and can only be settled by gross physical delivery of shares or net share settlement, the Company has determined that this warrant qualifies for equity classification. Therefore, the warrant was recorded as a credit to additional paid-in capital and as a debt discount on the date of issuance. Based on the relative fair values of the warrant and the notes payable on the date of the grant, the amount allocated to the warrant was $5, determined using the Black-Scholes option-pricing model with the following inputs: ten-year expected term; 2.41% risk-free interest rate; 70.93% expected volatility; and no dividends. The debt discount at date of issuance, which also included $30 of fees paid to the lender and for legal services, is being accreted to the carrying value of the debt using the effective interest method. In December 2015, the Company, in connection with the formation of Dimension Securities Corporation, entered into a First Amendment to the loan and security agreement with SVB. The First Amendment requires the Company to maintain on deposit in operating, depository and securities accounts maintained with SVB a compensating balance of 105% of the then-outstanding obligations, as defined in the loan and security agreement. As December 31, 2016, the Company was required to maintain a compensating balance of $6,844 in its accounts with SVB to meet this requirement. In February 2016, the Company entered into a Second Amendment to the loan and security agreement (the “Second Amendment”) with Silicon Valley Bank (“SVB”). Under the Second Amendment, the Company may borrow an aggregate principal amount of up to $7,000 for certain equipment purchases. In accordance with terms of the Second Amendment, the Company is obligated to make six monthly interest-only payments. Thereafter, the Company is obligated to pay thirty-six (36) monthly installments of interest and principal. Advances under the second amendment to loan and security agreement accrue interest at an annual rate of 0.5% below the Prime Rate, which interest shall be determined by SVB on the Funding Date of the applicable Equipment Advance and shall be payable monthly in arrears. In addition, a final payment equal to 6% of the original principal amount of each advance is due on the earlier of the maturity date of the advance, acceleration of the terms of the loan, prepayment of the advance or termination of the loan and security agreement. Through December 31, 2016, the Company borrowed $5,801 under the Second Amendment in three advances and incurred $22 of related issuance costs. The issuance costs are being accreted to the principal amount of debt and being amortized to interest expense using the effective-interest method over the same period. In accordance with the terms of the Second Amendment, the borrowed loan amounts through December 31, 2016, accrues interest at an annual rate of 3.0%. In addition, the Company accrues the related total final payments due of $348 to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the maturity date. The effective interest rate of the outstanding debt under the Second Amendment is approximately 6.2%. As of December 31, 2016 and December 31, 2015, notes payable consist of the following: December 31, December 31, 2016 2015 Notes payable $ 6,478 $ 1,356 Less: current portion (2,361 ) (574 ) Notes payable, net of current portion 4,117 782 Debt discount, net of accretion (30 ) (23 ) Accretion related to final payment 82 — Notes payable, net of discount, long term $ 4,169 $ 759 During the years ended December 31, 2016 and 2015, the Company made scheduled principal repayments totaling $679 and $394, respectively. During the years ended December 31, 2016 and 2015, the Company recognized $217 and $88, respectively, of interest expense related to the loan and security agreement with SVB. Borrowings under the loan and security agreement are secured by substantially all of our assets, except for our intellectual property. There are no financial covenants associated with the loan and security agreement. There are negative covenants restricting our activities, such as disposing of our business or certain assets, changing our business, management, ownership or business locations, incurring additional debt or liens or making payments on other debt, making certain investments and declaring dividends, acquiring or merging with another entity, engaging in transactions with affiliates or encumbering intellectual property. The obligations under the loan and security agreement are subject to acceleration upon occurrence of specified events of default, including a material adverse change in our business, operations or financial or other condition. Estimated future principal payments due under the loan and security agreement with SVB are as follows: Year Ended December 31, 2017 $ 2,379 2018 2,123 2019 1,838 2020 138 Total $ 6,478

Collaboration and License Agree

Collaboration and License Agreements	12 Months Ended
	Dec. 31, 2016
Collaboration And License Agreements [Abstract]
Collaboration and License Agreements	8. Collaboration and License Agreements Bayer HealthCare LLC In June 2014, the Company entered into a research and development collaboration and license agreement with Bayer for the development and commercialization of a gene therapy for the treatment of hemophilia A (the “collaboration agreement”). Under the agreement, the Company granted to Bayer an exclusive license to develop and commercialize one or more novel gene therapies for hemophilia A. The Company is responsible for all research and development activities for those biologic therapeutics under the agreement prior to completion of clinical proof-of-concept (“POC”) trials, with Bayer funding the Company’s efforts and providing consulting and technological support. Upon the successful demonstration of clinical POC, the agreement requires that Bayer use commercially reasonable efforts to conduct later-stage development and commercialization of gene therapy products for treatment of hemophilia A for such therapy. Bayer will have worldwide rights to commercialize the potential future product. Under this agreement, Bayer was also granted a right of first notice for gene therapy treatments for hemophilia B, which it declined to exercise in July 2016. Financial Terms Under the terms of the agreement with Bayer, the Company received a nonrefundable, noncreditable upfront license payment of $20,000 in June 2014 and is eligible to receive development and commercialization milestone payments of up to $232,000, as well as tiered royalty payments ranging in the high single-digit to low double-digit percentages, not exceeding the mid-teens, of net sales of commercialized products resulting from the collaboration, as defined in the agreement with Bayer. Bayer will fund certain research and development services performed by the Company during the research term and will reimburse the Company for all project costs, including any third-party costs, in the performance of its obligations under the annual research plan and in accordance with the mutually agreed upon research budget. The Company determined that the deliverables under its agreement with Bayer include (i) research services to be provided over the research term, (ii) a development and commercialization license and (iii) the Company’s participation in a Joint Steering Committee (“JSC”) and a Joint Research and Development Committee (“JRDC”) to be provided over the research term. The Company determined that the development and commercialization license and involvement in the JSC and the JRDC do not have standalone value to Bayer and, therefore, are not separable from the delivery of the research services. Therefore, all deliverables under the agreement have been combined and accounted for as a single unit of accounting. Accordingly, the upfront license payment and research payments are being recognized by the Company as revenue on a straight-line basis over the estimated performance period, which approximates the 54-month research term of the agreement with Bayer that commenced in June 2014. As future research payments are earned, the Company will recognize as revenue the portion of payments equal to the percentage of the elapsed research term to the total estimated research term, with the remaining portion of consideration received being recognized over the remaining estimated performance period on a straight-line basis. The Company concluded at the outset of the arrangement that none of the future milestone payments included in the arrangement qualified as substantive milestones. Any future milestone payments will be recognized, along with the other arrangement consideration, over the remaining estimated period of performance, if any, beginning at the time a milestone payment is earned, with a cumulative catch-up being recognized for the elapsed portion of the estimated research term. As of December 31, 2016, the Company had not earned any milestone or royalty payments. Revenue recognized under the collaboration agreement with Bayer consisted of the following: December 31, 2016 2015 2014 Revenue: Bayer Collaboration Agreement $ 11,471 $ 7,750 $ 2,750 $ 11,471 $ 7,750 $ 2,750 During the years ended December 31, 2016, 2015 and 2014, research payments earned by the Company under the collaboration agreement totaled $8,292, $7,740 and $3,265, respectively. The costs incurred by the Company related to the research activities of the collaboration agreement are recorded as research and development expense in the consolidated statement of operations and comprehensive loss. As of December 31, 2016 and 2015, deferred revenue related to the collaboration agreement with Bayer totaled $17,326 and $20,505, which related to the upfront license payment, research services payments and to advance payments for future research services. Term and Termination The agreement with Bayer expires on a licensed treatment-by-licensed treatment and country-by-country basis until the later of ten years from the date of first commercial sale or when patent claims have expired, lapsed, been abandoned or been invalidated in such country. The Company and Bayer, through the JSC, may mutually agree to terminate the collaboration early in the event clinical development is ended as required by a regulatory authority or in light of data from any studies conducted prior the POC trial. Bayer has the unilateral right to terminate the agreement in its entirety or with respect to one or more country by providing written notice to the Company. Bayer may also terminate the agreement with notice after two unsuccessful attempts to demonstrate clinical POC in a Phase I human POC trial. Bayer may further terminate the agreement with notice in the event that, following the POC trial, there is a material safety issue with respect to a licensed product. Reciprocal termination rights under the agreement include termination for breach and termination for bankruptcy. REGENX 2013 License Agreement In October 2013, the Company entered into a license agreement with REGENX, as amended in June and September 2014 (the "REGENX 2013 License Agreement") for an exclusive, sublicensable, worldwide commercial license under certain intellectual property for preclinical and clinical research and development, and commercialization of drug therapies using REGENX's licensed patents for the treatment of hemophilia A and hemophilia B, as well as a one-year option to obtain exclusive licenses for the commercialization of two other diseases to be elected by the Company in the future. The REGENX 2013 License Agreement requires the Company to pay ongoing annual maintenance fees of $35 for each indication elected by the Company, beginning in October 2014. The Company is also required to pay low to mid single-digit royalties on net sales of licensed products as well as milestone fees, if any, owed by REGENX to Glaxo Smith Kline ("GSK") or sublicense fees owed by REGENX to Penn or GSK as a result of the Company's activities under the REGENX 2013 License Agreement. The milestones payable will not exceed $1,650 and are payable contingent not only on the Company achieving the milestone, but also on the Company achieving the milestone before all other licensees. The REGENX 2013 License Agreement expires upon the expiration, lapse, abandonment or invalidation of the last claim of the licensed intellectual property to expire, lapse or become abandoned or unenforceable in all the countries of the world. Upon expiration, the Company's knowhow license will become non-exclusive, perpetual, irrevocable and royalty-free with respect to licensed know-how that REGENX owns in the field and will continue with respect to all of REGENX's other know-how in the field under its GSK and/or Penn licenses for so long as its rights from those licensors continue. The Company may terminate REGENX License Agreement upon prior written notice to REGENX. REGENX may terminate the license agreement if the Company or its affiliates become insolvent, if the Company is greater than a specified number of days late in paying money due under the license agreement, or, effective immediately, if the Company or its affiliates commence certain actions relating to the licensed patents. Either party may terminate the REGENX 2013 License Agreement for a material breach that is not cured within a specified number of days. In September 2014, the Company elected its third indication under the REGENX 2013 License Agreement, and the license was amended to extend the term of the option to elect the fourth and final disease indication for an additional six months. In consideration for the extension of the option, the Company paid an extension fee of $150. In January 2015, the Company elected its fourth and final indication under the REGENX 2013 License Agreement. As of December 31, 2016 and 2015, the Company had not developed a commercial product using the licensed technologies and no milestones had been achieved. Research and Management Services Agreement In October 2013, in connection with the REGENX 2013 License Agreement, the Company engaged REGENX for the provision of certain research services (the "REGENX Research Services Agreement") through REGENX's existing sponsored research agreement with Penn. Management Services Agreement In April 2014, the Company engaged REGENX Holdings LLC for the provision of certain research and development management services (the "REGENX Management Services Agreement"), including strategic, business and operational advice. Material Transfer Agreement In April 2014, the Company entered into a material transfer agreement and purchase order with REGENX, pursuant to which the Company purchased certain biologic materials from REGENX for research and development and clinical trials. 2015 Option and License Agreement In March 2015, the Company entered into an option and license agreement with REGENX (the "REGENX 2015 Option and License Agreement") that granted the Company options to commercial exclusive licenses for four new disease indications to be elected by the Company in the future. If elected, each option carries an option fee of $1,000 payable to REGENX upon exercise and annual maintenance fees of $50. In addition, for each option exercised, the Company is obligated to pay REGENX up to $9,000 upon achievement of various substantive milestones, as well as mid to high single-digit royalties on net sales of licensed products and mid single-digit to low double-digit percentage sublicenses fees, if any. In May, August and December 2015, the Company exercised options under the REGENX 2015 Option and License Agreement for three new indications and paid a $1,000 option fee for each indication pursuant to the agreement. Any options not exercised will automatically terminate in 2019 (which may be extended for additional time for a fee). The Company may terminate the REGENX 2015 Option and License Agreement upon prior written notice. REGENX may terminate the agreement if the Company or its controlling affiliates become insolvent, if the Company is greater than a specified number of days late in paying money due under the REGENX 2015 Option and License Agreement, or, effective immediately, if the Company or its affiliates commence certain actions relating to the licensed patents. Either party may terminate the REGENX 2015 Option and License Agreement for a material breach that is not cured within a specified number of days. University of Pennsylvania 2015 Sponsored Research and Option Agreement In January 2015, the Company entered into an agreement with the Penn to sponsor certain research of Dr. Wilson at University of Pennsylvania School of Medicine related to liver gene therapy and hemophilia. The agreement was subsequently amended in August 2015, March 2016 and December 2016. In consideration for funding such research, Penn granted the Company an option to obtain a worldwide, non-exclusive or exclusive, royalty-bearing license, with the right to sublicense, under certain patent rights conceived, created or reduced to practice in the conduct of the research. The Company is required to reimburse Penn for filing, prosecuting and maintaining such patent rights unless and until the Company declines to exercise its option. Dr. Wilson is required to provide the Company with task-based, scientific reports of progress and results of the research, and Penn granted the Company a royalty-free, nontransferable, non-exclusive right to copy and distribute any research reports furnished to the Company for any reasonable purpose, provided the results are not made publicly available until certain conditions are met, and the right to use, disclose and otherwise exploit the research results for any reasonable purpose, subject to similar restrictions on our public disclosure of the research results. Otherwise, the sponsored research agreement contains customary confidentiality provisions. The Company’s sponsored research agreement with Penn will expire on December 31, 2017 unless earlier terminated or the parties mutually agree to extend or renew the agreement. Either party may terminate the agreement if Dr. Wilson becomes unavailable and an acceptable substitute is not found within a certain period of time, or if the Company fails to mutually agree on an acceptable work plan and budget for the sponsored research. The Company may also terminate the sponsored research agreement upon written notice to Penn, as long as the Company has met all of its payment and performance obligations to date. In the event of termination, the Company shall pay Penn the amount needed to cover costs through the effective termination date as well as allowable commitments incurred; provided, however, payments for allowable commitments extending beyond the effective termination date shall not exceed $2,762, as of December 31, 2016. Either party may terminate this agreement for a material breach that is not cured within a specified number of days. 2016 Research, Collaboration and License Agreement In May 2016, the Company entered into a research, collaboration and license agreement with Penn. The agreement was subsequently amended in October 2016 and December 2016. The agreement provides the terms for the Company and Penn to collaborate with respect to the pre-clinical development of gene therapy products for the treatment of citrullinemia type I (DTX601), phenylketonuria (DTX501) and Wilson disease (DTX701) (each, a “Subfield”). Under the agreement, Penn granted the Company an exclusive, worldwide, royalty-bearing right and license to certain patent rights arising out of the research program, subject to certain retained rights, and a non-exclusive, worldwide, royalty-bearing right and license to certain Penn intellectual property, in each case to research, develop, make, have made, use, sell, offer for sale, commercialize and import licensed products in each Subfield for the term of the agreement. In May 2016, the Company paid to Penn immaterial one-time, non-refundable upfront payments recorded as research and development expense in exchange for the licenses. The Company will fund the cost of the research program in accordance with a mutually agreed-upon research budget and will be responsible for clinical development, manufacturing and commercialization of each Subfield. Research program costs will be recorded as research and development expense in the period the research services are performed. In addition, the Company would be required to make milestone payments if certain development and commercial milestones are achieved over time, as well as low to mid single-digit royalties percentages on net sales of each Subfield’s licensed products. Should they be achieved, potential development milestones through U.S. and EU regulatory acceptance total up to a maximum of $5,000 per Subfield and would be recorded as research and development expense in the periods they are achieved. As no milestones have been achieved and the agreement can be cancelled at our option, no future potential milestone payments have accrued as of December 31, 2016.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions	9. Related Party Transactions In October 2013, the Company entered into a license agreement (see Note 8) with REGENX Biosciences, LLC, predecessor to REGENXBIO Inc., ("REGENX") and issued common stock and restricted common stock to REGENX and directors and management of REGENX as well as to Penn and an affiliate of Penn. In connection with the transaction, the Company issued, at a price of $0.0002923 per share, 3,421 shares of common stock to REGENX, 2,379,242 shares of common stock to directors and management of REGENX and 1,038,474 shares of common stock to Penn and an affiliate of Penn. In October 2013, the Company also entered into a Stock Purchase Agreement (the "SPA") with Beacon Bioventures Fund III Limited Partnership ("Beacon Bioventures") for the issuance of Series A preferred stock, pursuant to which Beacon Bioventures purchased 5,000,000 shares of the Company's Series A preferred stock in October 2013 and 10,000,000 shares of the Company's Series A preferred in February 2015, each at a price of $1.00 per share. In addition, in October 2013, the Company issued to Beacon Bioventures in exchange for consulting services provided by Fidelity Biosciences Corp. ("Fidelity Biosciences"), an entity affiliated with Beacon Bioventures, an aggregate of 153,951 shares of fully vested common stock as well as 85,528 shares of restricted common stock at a price of $0.0002923 per share, which shares were subject to repurchase by the Company from April 30, 2014 to October 27, 2014. The issuance of restricted common stock to Beacon Bioventures was considered an award to a non-employee requiring the fair value of these awards to be remeasured at the end of each reporting period prior to the completion of the consulting services, using the then-current fair value of the Company's common stock. Related to the issuance of the 85,528 shares of restricted common stock, the Company recorded general and administrative expenses of $42 for the year ended December 31, 2014. As of December 31, 2014, REGENX and directors and management of REGENX, Penn and an affiliate of Penn, and Beacon Bioventures were principal stockholders of the Company. In addition, as of the same date, each of REGENX and Beacon Bioventures was represented by at least one member of the Company's board of directors. As of December 31, 2015, REGENX and directors and management of REGENX, Penn and an affiliate of Penn, and Beacon Bioventures were principal stockholders of the Company. However, as of the same date, REGENX and Beacon Bioventures were no longer represented in the Company’s board of directors. As of January 1, 2016, the Company no longer considers the Penn or REGENX to be related parties, as Penn and REGENX were no longer principal shareholders and REGENX was no longer represented on the Company’s board of directors. Accordingly, expenses incurred in connection with Penn or REGENX subsequent to January 1, 2016 are no longer considered to be related party expenses. The Company entered into two license agreements with REGENX and two research agreements with University of Pennsylvania (see Note 8). Related party expenses consisted of the following: December 31, 2015 2014 Related Parties: REGENX - R&D Expense $ 3,140 $ 7,139 Penn - R&D Expense 5,659 — Fidelity Biosciences - G&A Expense — 45 $ 8,799 $ 7,184 Cash payments made to REGENXBIO, Inc. (“REGENX”) during the years ended December 31, 2015 and 2014 were $3,890, and $7,341, respectively. There were no related party amounts due to REGENX by the Company as of December 31, 2015. Cash payments made to the University of Pennsylvania during the years ended December 31, 2015 and 2014 were $5,137 and $0, respectively. Related party amounts due to University of Pennsylvania by the Company were $522 as of December 31, 2015. Cash payments made to Fidelity Biosciences during the years ended December 31, 2015 and 2014 were $1 and $18, respectively. There were no related party amounts due to Fidelity Biosciences by the Company as of December 31, 2015.

Stockholder's Equity

Stockholder's Equity	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Stockholder's Equity	10. Stockholders’ Equity On October 9, 2015, the Company effected a one-for-2.923 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Convertible Preferred Stock. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. The significant increase in common stock outstanding in October 2015 will impact the year-over-year comparability of the Company’s net loss per share calculations over the next year. On October 27, 2015, the Company completed an IPO, which resulted in the issuance and sale of 5,500,000 shares of its common stock at a public offering price of $13.00 per share, resulting in net proceeds of $63,368 after deducting underwriting discounts and other offering costs. Upon the closing of the IPO, all outstanding shares of the Company’s Convertible Preferred Stock were converted into 15,286,968 shares of the Company’s common stock. On November 25, 2015, the underwriters of the Company’s IPO exercised their over-allotment option to purchase an additional 104,775 shares of common stock at the initial public offering price of $13.00 per share, resulting in net proceeds of $1,252 after deducting underwriting discounts and other offering costs. Upon closing of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock were converted into 15,286,968 shares of common stock. As of December 31, 2016 and 2015, the Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 5,000,000 shares of $0.0001 par value undesignated preferred stock. The rights, preferences, restrictions, qualifications and limitations of such stock are to be determined by the Company’s board of directors. As of December 31, 2016 and 2015, the Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 150,000,000 shares of $0.0001 par value common stock. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company's stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any. Through December 31, 2016, no dividends have been declared.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	11. Stock-Based Compensation 2015 Stock Option and Incentive Plan On September 2, 2015, the Company’s stockholders approved the 2015 Stock Option and Incentive Plan (the “2015 Plan”), which became effective October 21, 2015. The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, performance share awards, cash-based awards and other stock-based awards. The 2015 Stock Option Plan replaced the Company’s 2013 Stock Option and Grant Plan (the “2013 Plan”). The Company will grant no future stock options or other awards under the 2013 Stock Option Plan. Any options or awards outstanding under the 2013 Stock Option Plan remained outstanding and effective. Vesting periods are determined at the discretion of the board of directors, the compensation committee of the board of directors or a similar committee performing the functions of the compensation committee and which is comprised of not less than two non-employee directors who are independent. Stock options granted to employees and directors typically vest over four years. Stock options granted to non-employees typically vest over periods ranging from two years to four years, depending on the period during which the services are provided. The Company measures and records the value of options granted to non-employees over the period of time services are provided and, as such, unvested portions are subject to remeasurement at subsequent reporting periods. During the year ended December 31, 2016 and 2015, under the 2015 Plan, the Company granted to employees and directors no shares of restricted common stock and options to purchase 1,192,814 The total number of shares that may be issued under all equity plans was 5,889,101, of which 1,223,561 shares remained available for future grant as of December 31, 2016. The number of shares of common stock that may be issued under the 2015 Plan will automatically increase on each January 1, beginning on January 1, 2016 and ending on January 1, 2020, equal to the lesser of (i) 4% of the shares of the Company’s common stock outstanding on the immediately preceding December 31 or (ii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, canceled, repurchased or are otherwise terminated by the Company under the 2015 Plan will be added back to the shares of common stock available for issuance under the 2015 Plan. On January 1, 2016, 1,000,329 shares of common stock, representing 4% of the Company’s issued and outstanding shares of common stock as of December 31, 2015, were added to the 2015 Plan. Such shares are included in the equity plan totals specified above. 2015 Employee Stock Purchase Plan On September 2, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective October 27, 2015. A total of 256,585 shares of common stock were reserved for issuance under this plan as of December 31, 2016. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on each January 1, beginning on January 1, 2017 and ending on January 1, 2021, equal to the lesser of (i) 256,585 shares of common stock, (ii) 1% of the Company’s shares of common stock outstanding on the immediately preceding December 31 or (iii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. As of December 31, 2016 there was no activity under the ESPP. Stock Option Valuation The fair value of each option granted to employees and directors during the years ended December 31, 2016, 2015 and 2014, under the Company’s stock option plans has been calculated on the date of grant using the following assumptions: Year Ended December 31, 2016 2015 2014 Expected dividend yield — — — Expected volatility 83 - 91% 72 - 87% 71 - 78% Risk free interest rate 1.0 - 2.1% 1.6 - 2.4% 1.7 - 2.7% Expected term 5.3 - 6.0 years 5.6 - 9.0 years 5.5 - 10 years Expected dividend yield: The Company has not paid and does not anticipate paying any dividends in the foreseeable future. Risk-free interest rate: The Company determined the risk-free interest rate by using a weighted average equivalent to the expected term based on the U.S. Treasury yield curve in effect as of the date of grant. Expected volatility: As the Company has only been a public company since October 2015, there is not sufficient historical volatility for the expected term of the options. Therefore, the Company used an average historical share price volatility based on an analysis of reported data for a peer group of comparable companies. Expected term (in years): Expected term represents the period that the Company’s share option grants are expected to be outstanding. As the Company has only been a public company since October 2015, there is not sufficient historical term data to calculate the expected term of the options. Therefore, the Company elected to utilize the “simplified” method to estimate the expected term of option grants issued to employees. Under this approach, the weighted average expected life is presumed to be the average of the vesting term and the contractual term of the option. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. The Company estimates forfeitures based on historical analysis of actual stock option forfeitures. For options granted to nonemployees, the expected life of the option used is ten years, which is the contractual term of each such option. All other assumptions used to calculate the grant date fair value are generally consistent with the assumptions used for options granted to employees. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2015: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Granted 1,192,814 $ 7.15 Exercised (35,279 ) $ 1.22 $ 110 Cancelled or forfeited (68,858 ) $ 4.36 Outstanding as of December 31, 2016 4,185,699 $ 4.26 8.4 $ 4,757 Options vested and expected to vest as of December 31, 2016 4,185,699 $ 4.26 8.4 $ 4,757 Options exercisable as of December 31, 2016 1,553,567 $ 3.14 8.1 $ 2,644 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock at December 31, 2016. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2016 and 2015 was $7.15 and $4.49 per share, respectively. As of December 31, 2016 there were no outstanding unvested service-based stock options held by non-employees for purchase. As of December 31, 2015, there were outstanding unvested service-based stock options held by non-employees for the purchase of 3,527 shares of common stock. Until April 2015, the Company had been obligated to issue additional stock options to purchase shares of the Company's common stock to certain executives of the Company such that these executives would maintain their potential stock ownership in the event that the Company granted additional shares of Series A preferred stock. In connection with the Company's issuance of Series A preferred stock in February 2015, the Company granted stock options in April 2015 for the purchase of 102,086 shares of common stock to those executives. Restricted Common Stock The Company has granted restricted common stock with time-based vesting conditions. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The following table summarizes the Company’s restricted common stock activity since December 31, 2014: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted common stock at December 31, 2015 153,217 $ 0.87 Issued — $ — Vested (83,164 ) $ 0.85 Unvested restricted common stock at December 31, 2016 70,053 $ 0.88 The total fair value of restricted common stock vested during the years ended December 31, 2016 and 2015 was $362 and $286, respectively. Stock-Based Compensation Stock-based compensation expense related to stock options and restricted common stock was classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2016 2015 2014 Research and development $ 1,147 $ 664 $ 46 General and administrative 2,220 669 40 $ 3,367 $ 1,333 $ 86 As of December 31, 2016, total unrecognized compensation cost related to the unvested awards was $7,930, which is expected to be recognized over a weighted average period of 2.6 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes During the years ended December 31, 2016 and 2015, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items. All of the Company's losses before income taxes were generated in the United States. A reconciliation of U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2016 Year Ended December 31, 2015 Year Ended December 31, 2014 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% State income taxes, net of federal benefit (5.1 ) (5.1 ) (5.2 ) Tax credits (6.0 ) (2.6 ) (4.1 ) Nondeductible expenses paid in common stock 2.0 0.9 0.3 Change in deferred tax asset valuation allowance 43.1 40.8 43.0 Effective income tax rate 0.0 % 0.0 % 0.0 % Net deferred tax assets as of December 31, 2016 and 2015 consisted of the following: December 31, 2016 2015 Deferred tax assets: Net operating loss carryforwards $ 28,705 $ 9,808 Deferred revenue 6,806 8,055 Tax credit carryforwards 4,410 1,480 Capitalized start-up costs and other 907 193 Intangible Assets 1,071 1,150 Total deferred tax assets 41,899 20,686 Deferred tax liabilities: Depreciation and amortization (373 ) (277 ) Total deferred tax liabilities (373 ) (277 ) Valuation allowance (41,526 ) (20,409 ) Net deferred tax assets $ — $ — As of December 31, 2016, the Company had net operating loss carryforwards for federal and state income tax purposes of $73,099 and $73,498, respectively, which begin to expire in 2033. The Company also had an additional $74 of federal and state net operating losses not reflected above that were attributable to stock option exercises, which will be recorded as an increase in additional paid-in capital once they are realized in accordance with accounting for stock-based compensation awards. As of December 31, 2016, the Company also had available research and development and orphan drug tax credit carryforwards for federal and state income tax purposes of $3,785 and $625 respectively, which begin to expire in 2033 and 2029, respectively. Utilization of the net operating loss carryforwards and research and development and orphan drug tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. Utilization of the net operating loss carryforwards and research and development and orphan drug tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development and orphan drug tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company's stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development and orphan drug tax credit carryforwards before utilization. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company's history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2016, 2015 and 2014. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2016, 2015 and 2014, related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and development and orphan drug tax credit carryforwards and were as follows: Year Ended December 31, 2016 Year Ended December 31, 2015 Year Ended December 31, 2014 Valuation allowance at beginning of year $ (20,409 ) $ (6,120 ) $ (544 ) Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision (21,117 ) (14,289 ) (5,576 ) Valuation allowance at end of year $ (41,526 ) $ (20,409 ) $ (6,120 ) The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2016, 2015 and 2014. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s tax years are still open under statute from 2013 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Loss per Share	13. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2016 2015 2014 Numerator: Net loss $ (49,001 ) $ (35,056 ) $ (12,968 ) Accretion of convertible preferred stock to redemption value — (23 ) (71 ) Net loss attributable to common stockholders $ (49,001 ) $ (35,079 ) $ (13,039 ) Denominator: Weighted average common shares outstanding — basic and diluted 24,801,610 8,138,629 3,977,274 Less: Weighted average shares of unvested restricted common stock outstanding (114,277 ) (188,959 ) (366,682 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 24,915,887 7,949,670 3,610,592 Net loss per share attributable to common stockholders — basic and diluted $ (1.97 ) $ (4.41 ) $ (3.61 ) The Company’s potential dilutive securities, which include convertible preferred stock, stock options, unvested restricted common stock and a warrant to purchase common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an antidilutive effect: December 31, 2016 2015 2014 Convertible preferred stock (as converted to common stock) — — 3,421,142 Options to purchase common stock 4,185,699 3,097,022 1,104,172 Unvested restricted common stock 70,053 153,217 210,287 Warrant for the purchase of common stock — — 11,973 4,255,752 3,250,239 4,747,574

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	14. Commitments and Contingencies Leases The Company leases approximately 15,000 square foot office and laboratory space under various operating lease agreements for its Cambridge, MA location that, as amended, expire in January 2018. In connection with these agreements, in March 2014, the Company made security deposits totaling $88. In October 2014, one of the leases was amended to increase the leased space and to extend the term of the lease through January 2018. In connection with the lease amendment, the Company entered into an equipment-collateralized irrevocable standby letter of credit (see Note 5) in the amount of $81, naming the landlord as beneficiary. In March 2015, the Company entered into a second equipment-collateralized irrevocable standby letter of credit (see Note 5) in the amount of $92, naming the landlord as beneficiary, which replaced the security deposits paid by the Company in March 2014. In April 2015, the security deposits of $88 were returned to the Company. As amended, required lease payments include base rent of $53 per month through March 2015, which increases to $62 per month over the course of the amended lease through January 2018. In November 2015, the Company entered into an operating lease agreement with ARE-MA Region No. 20, LLC (“ARE”) for approximately 17,600 square feet of laboratory and office space located at 19 Presidential Way, Woburn, Massachusetts. The lease commenced upon substantial completion of building improvements on April 1, 2016 and continues for a term of 5 years. Annual rent under the lease, exclusive of operating expenses and real estate taxes, will be $342 for the first year, with annual increases of $1.00 per square foot each year thereafter. Under the terms of the lease, ARE contributed $35.00 per rentable square foot of tenant improvement allowances, which was included in the base rent set forth in the lease. The ARE lease contains customary provisions allowing ARE to terminate our lease should the Company fail to remedy any breach of our obligations within specified time periods, or upon our bankruptcy or insolvency. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. The Company recorded rent expense of $960, $734 and $227 during the years ended December 31, 2016, 2015 and 2014, respectively. The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases: Year Ending December 31, 2017 1,205 2018 550 2019 505 2020 523 2021 132 Total $ 2,915 The Company is further responsible for its pro rata share of operating expenses, real estate taxes and utilities of the facilities housing the leased office and laboratory space. License Agreements The Company has entered into license agreements with REGENX and the University of Pennsylvania under which it is obligated to make contingent payments (see Note 8). Research and Development Agreement In June 2015, the Company entered into a three-year cooperative research and development agreement with the Eunice Kennedy Shriver National Institute of Child Health and Human Development, an institute of the U.S. National Institutes of Health, to conduct on behalf of the Company a non-clinical study to investigate the effectiveness of a potential gene therapy treatment for patients with GSDIa. Under the agreement, as amended in November 2016, the Company is obligated to make noncancelable research funding payments of $244 for the first year and $300 annually for the remaining two years, for total aggregate payments of $844. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to its legal proceedings. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2016.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2016
Postemployment Benefits [Abstract]
401(k) Savings Plan	15. 401(k) Savings Plan The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Company contributions to the plan may be made at the discretion of the Company's board of directors. To-date, no contributions have been made to the plan by the Company.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	16. Quarterly Financial Data (Unaudited) The following tables present quarterly consolidated statement of operations data for fiscal 2016 and 2015. The below data is unaudited but, in our opinion, reflects all adjustments necessary for a fair presentation of this data in accordance with GAAP. Three Months Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (unaudited) Revenue $ 2,206 $ 2,371 $ 2,762 $ 4,132 Operating expenses 11,746 14,476 17,101 17,198 Net loss attributable to common stockholders $ (9,512 ) $ (12,099 ) $ (14,365 ) $ (13,027 ) Net loss per share attributable to common stockholders — basic and diluted $ (0.38 ) $ (0.49 ) $ (0.58 ) $ (0.52 ) Weighted average shares outstanding — basic and diluted 24,851,933 24,899,479 24,930,348 24,960,899 Three Months Ended March 31, 2015 June 30, 2015 September 30, 2015 December 31, 2015 (unaudited) Revenue $ 1,594 $ 1,742 $ 2,061 $ 2,352 Operating expenses 6,959 11,688 12,706 11,361 Net loss attributable to common stockholders $ (5,408 ) $ (9,976 ) $ (10,667 ) $ (9,028 ) Net loss per share attributable to common stockholders — basic and diluted $ (1.40 ) $ (2.57 ) $ (2.73 ) $ (0.45 ) Weighted average shares outstanding — basic and diluted 3,853,262 3,881,288 3,907,196 20,023,659

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements reflect the operations of Dimension Therapeutics, Inc. and Dimension Securities Corporation, our wholly-owned subsidiary. All intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, revenue recognition, the accrual for research and development expenses and the valuation of common stock. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.
Concentrations of Credit Risk, Significant Customers and Significant Suppliers	Concentrations of Credit Risk, Significant Customers and Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities and accounts receivable. The Company has all cash, cash equivalents and marketable securities balances at one accredited financial institution in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. As of December 31, 2016 and 2015 the Company's accounts receivable balances resulted from its collaboration agreement with Bayer (see Note 8), and the Company believes Bayer to be creditworthy. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of an equity financing, these costs are recorded in stockholders' equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. The Company has recorded deferred offering costs of $145 as of December 31, 2016 related to its At-the-Market (ATM) agreement entered into in connection with its Form S-3 Registration Statement prospectus dated November 10, 2016. Should shares under the AMT no longer be considered probable, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive loss. The Company did not record any deferred offering costs as of December 31, 2015.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents, which consist of money market funds and U.S. government securities, are stated at fair value.
Marketable Securities	Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated over their estimated useful lives using the straight-line method over the estimated useful life of each asset. Computer equipment is depreciated over five years. Laboratory equipment is depreciated over five years. Furniture and fixtures are depreciated over seven years. Leasehold improvements are amortized over the shorter of the lease term or the five-year estimated useful life of the asset. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Repairs and maintenance costs are expensed as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.
Fair Value Measurements	Fair Value Measurements Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 — Quoted prices in active markets for identical assets or liabilities. • Level 2 — Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.
Debt Issuance Costs	Debt Issuance Costs The Company has adopted the amendments to the Financial Accounting Standards Board ("FASB") Accounting Standards Codification made in Accounting Standards Update ("ASU") 2015-03, Interest — Imputation of Interest (Subtopic 835-30) ("ASU 2015-03") simplifying the presentation of debt issuance costs. According to ASU 2015-03, the Company presents debt issuance costs related to a recognized debt liability as a direct reduction from the carrying amount of that debt liability.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. All of the Company's tangible assets are held in the United States. To date, all of the Company's revenue has been generated in the United States.
Revenue Recognition	Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition. Accordingly, the Company recognizes revenue for each unit of accounting 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. Collaborative Research and License Agreements The terms of these agreements contain multiple deliverables, which may include licenses and research and development activities. The terms of these agreements may also include nonrefundable upfront license fees, payments for research and development activities, reimbursement of certain third-party costs, payments based upon the achievement of specified milestones, and royalty payments based on product sales derived from the collaboration. The Company evaluates its collaborative agreements for proper classification in its statements of operations and comprehensive loss based on the nature of the underlying activity. The Company generally reflects as revenue amounts due under its collaborative agreements related to reimbursement of development activities as the Company is generally the principal under the arrangement. The Company evaluates multiple-element arrangements based on the guidance in ASC Topic 605-25, Revenue Recognition — Multiple-Element Arrangements ("ASC 605-25"). Pursuant to the guidance in ASC 605-25, the Company evaluates multiple-element arrangements to determine (i) the deliverables included in the arrangement and (ii) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit. When the Company determines that an arrangement should be accounted for as a single unit of accounting, the Company must determine the period over which the performance obligations will be performed and revenue will be recognized. This evaluation requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that (i) the delivered item has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return with respect to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company's control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, 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 any other deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item, and whether there are other vendors that can provide the undelivered items. The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the selling price of a unit of accounting within each arrangement 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 as 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. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the contractual or estimated performance period for the undelivered items, which is typically the term of the Company's research and development obligations. If there is no discernible pattern of performance 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. Conversely, if the pattern of performance over 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. 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. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations. 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: (i) the consideration is commensurate with either the Company's performance to achieve the milestone or the enhancement of the value of the delivered item as a result of a specific outcome resulting from the Company's performance to achieve the milestone, (ii) the consideration relates solely to past performance, and (iii) 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 particular milestone and the level of effort and investment required to achieve the particular 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. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up being recognized for the elapsed portion of the period of performance, assuming all other revenue recognition criteria are met. To date, the Company has not recorded any substantive milestones because no milestones that meet the required criteria listed above have been identified. Payments for achievement of non-substantive milestones are deferred and recognized as revenue over the estimated period of performance applicable to the collaborative arrangement. As these milestones are achieved, the Company will recognize as revenue a portion of the milestone payment that is equal to the percentage of the performance period completed when the milestone is achieved, multiplied by the amount of the milestone payment, upon achievement of such milestone. The Company will recognize the remaining portion of the milestone payment over the remaining performance period under either the proportional performance method or on a straight-line basis. Amounts received prior to satisfying the revenue recognition criteria listed above are recorded as deferred revenue in the accompanying balance sheets. Although the Company follows detailed guidelines in measuring revenue, certain judgments affect the application of the Company's revenue policy. For example, in connection with its existing collaboration agreement, the Company has recorded on its balance sheet short-term and long-term deferred revenue based on its best estimate of when such revenue will be recognized. Short-term deferred revenue consists of amounts that are expected to be recognized as revenue in the next 12 months. Amounts that the Company expects will not be recognized within the next 12 months are classified as long-term deferred revenue. However, this estimate is based on the Company's current research plan and, if its research plan should change in the future, the Company may recognize a different amount of deferred revenue over the following 12-month period. The estimate of deferred revenue also reflects management's estimate of the periods of the Company's involvement in its collaboration. The Company's performance obligations under this collaboration consist of participation on steering committees and performance of other research and development services. In certain instances, the timing of satisfying these obligations can be difficult to estimate. Accordingly, the Company's estimates may change in the future. Such changes to estimates would result in a change in prospective revenue recognition amounts. If these estimates and judgments change over the course of any of the Company's collaborative agreements, it may affect the timing and amount of revenue that the Company will recognize and record in future periods.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third party license fees, and external costs of outside vendors engaged to conduct preclinical and clinical development activities and trials. Research and development expenses include the Company's costs of performing services in connection with its collaboration agreement with Bayer. Nonrefundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company's estimates. The Company's historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-Based Compensation	Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards with only service based vesting conditions and records the expense for these awards using the straight-line method. For stock-based awards granted to consultants and non-employees, compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company's common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its statement of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified. The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company's estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Prior to the Company’s initial public offering in October 2015, the Company was a private company and lacked company-specific historical and implied volatility information. Therefore, the Company estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company's stock options has been determined utilizing the "simplified" method for awards that qualify as "plain-vanilla" options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders' equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2016, the Company’s only element of other comprehensive loss was unrealized loss on marketable securities. For the years ended December 31, 2105 and 2014, there were no differences between net loss and comprehensive loss.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company's tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Net Loss per Share	Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options, warrants and unvested restricted stock. Net loss per share attributable to common stockholders is calculated using the two-class method, which is an earnings allocation formula that determines net loss per share for the holders of the Company's common shares and participating securities. The Company's convertible preferred stock contains participation rights in any dividend paid by the Company and are deemed to be participating securities. Net loss attributable to common stockholders and participating preferred shares are allocated to each share on an as-converted basis as if all of the earnings for the period had been distributed. The participating securities do not include a contractual obligation to share in losses of the Company and are not included in the calculation of net loss per share in the periods that have a net loss. Diluted net loss per share is computed using the more dilutive of (a) the two-class method or (b) the if-converted method. The Company allocates earnings first to preferred stockholders based on dividend rights, and then to common and preferred stockholders based on ownership interests. The weighted average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options, warrants, unvested restricted common stock and convertible preferred stock. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since dilutive common shares are not assumed to have been issued if their effect is antidilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting

Fair Value Measurements and M25

Fair Value Measurements and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2016 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 22,988 $ — $ — $ 22,988 U.S. government securities — 7,246 — 7,246 Total cash equivalents 22,988 7,246 — 30,234 Marketable securities: Commercial paper — 15,814 — 15,814 Corporate Bonds — 13,671 — 13,671 U.S. government securities — 18,230 — 18,230 Total marketable securities — 47,715 — 47,715 Total cash equivalents and marketable securities $ 22,988 $ 54,961 $ — $ 77,949 Fair Value Measurements as of December 31, 2015 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 127,047 $ — $ — $ 127,047 Total cash equivalents $ 127,047 $ — $ — $ 127,047
Summary of Marketable Securities	The following tables summarize the Company’s marketable securities as of December 31, 2016 Fair Value Measurements as of December 31, 2016 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 15,814 $ — $ — $ 15,814 Corporate bonds (due within 1 year) 9,182 — (10 ) 9,172 U.S. government securities (due within 1 year) 7,002 — — 7,002 Corporate bonds (due after 1 year through 2 years) 4,512 — (12 ) 4,500 U.S. government securities (due after 1 year through 2 years) 11,256 — (29 ) 11,227 $ 47,766 $ — $ (51 ) $ 47,715

Prepaid Expenses and Other Cu26

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: December 31, 2016 2015 Prepaid clinical, manufacturing, and scientific costs $ 4,406 $ 1,824 Prepaid insurance 637 587 Other 441 329 $ 5,484 $ 2,740

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, net	Property and Equipment, net consisted of the following: December 31, Useful Life 2016 2015 Laboratory equipment 5 years $ 8,114 $ 3,663 Computer equipment 5 years 251 165 Furniture and fixtures 7 years 310 193 Leasehold improvements * 2,259 198 10,934 4,219 Less: Accumulated depreciation and amortization (2,532 ) (880 ) $ 8,402 $ 3,339 * Shorter of asset life or lease term

Accrued Expenses and Other Cu28

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2016 2015 Payroll and payroll-related costs $ 2,234 $ 2,000 External research and development services 4,409 1,389 Professional fees 433 254 Other 171 72 $ 7,247 $ 3,715

Notes Payable (Tables)

Notes Payable (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Summary of Notes Payable	As of December 31, 2016 and December 31, 2015, notes payable consist of the following: December 31, December 31, 2016 2015 Notes payable $ 6,478 $ 1,356 Less: current portion (2,361 ) (574 ) Notes payable, net of current portion 4,117 782 Debt discount, net of accretion (30 ) (23 ) Accretion related to final payment 82 — Notes payable, net of discount, long term $ 4,169 $ 759
Estimated Future Principal Payments Due	Estimated future principal payments due under the loan and security agreement with SVB are as follows: Year Ended December 31, 2017 $ 2,379 2018 2,123 2019 1,838 2020 138 Total $ 6,478

Collaboration and License Agr30

Collaboration and License Agreements (Tables)	12 Months Ended
	Dec. 31, 2016
Collaboration And License Agreements [Abstract]
Schedule of Revenue from Collaboration Agreement	Revenue recognized under the collaboration agreement with Bayer consisted of the following: December 31, 2016 2015 2014 Revenue: Bayer Collaboration Agreement $ 11,471 $ 7,750 $ 2,750 $ 11,471 $ 7,750 $ 2,750

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Summary of Related Party Expenses	Related party expenses consisted of the following: December 31, 2015 2014 Related Parties: REGENX - R&D Expense $ 3,140 $ 7,139 Penn - R&D Expense 5,659 — Fidelity Biosciences - G&A Expense — 45 $ 8,799 $ 7,184

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Assumptions Used in Fair Value of Options Granted to Employees and Directors Under Stock Option Plans	The fair value of each option granted to employees and directors during the years ended December 31, 2016, 2015 and 2014, under the Company’s stock option plans has been calculated on the date of grant using the following assumptions: Year Ended December 31, 2016 2015 2014 Expected dividend yield — — — Expected volatility 83 - 91% 72 - 87% 71 - 78% Risk free interest rate 1.0 - 2.1% 1.6 - 2.4% 1.7 - 2.7% Expected term 5.3 - 6.0 years 5.6 - 9.0 years 5.5 - 10 years
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2015: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Granted 1,192,814 $ 7.15 Exercised (35,279 ) $ 1.22 $ 110 Cancelled or forfeited (68,858 ) $ 4.36 Outstanding as of December 31, 2016 4,185,699 $ 4.26 8.4 $ 4,757 Options vested and expected to vest as of December 31, 2016 4,185,699 $ 4.26 8.4 $ 4,757 Options exercisable as of December 31, 2016 1,553,567 $ 3.14 8.1 $ 2,644
Summary of Restricted Common Stock Activity	The following table summarizes the Company’s restricted common stock activity since December 31, 2014: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted common stock at December 31, 2015 153,217 $ 0.87 Issued — $ — Vested (83,164 ) $ 0.85 Unvested restricted common stock at December 31, 2016 70,053 $ 0.88
Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	Stock-based compensation expense related to stock options and restricted common stock was classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2016 2015 2014 Research and development $ 1,147 $ 664 $ 46 General and administrative 2,220 669 40 $ 3,367 $ 1,333 $ 86

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Summary of Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate	A reconciliation of U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2016 Year Ended December 31, 2015 Year Ended December 31, 2014 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% State income taxes, net of federal benefit (5.1 ) (5.1 ) (5.2 ) Tax credits (6.0 ) (2.6 ) (4.1 ) Nondeductible expenses paid in common stock 2.0 0.9 0.3 Change in deferred tax asset valuation allowance 43.1 40.8 43.0 Effective income tax rate 0.0 % 0.0 % 0.0 %
Summary of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2016 and 2015 consisted of the following: December 31, 2016 2015 Deferred tax assets: Net operating loss carryforwards $ 28,705 $ 9,808 Deferred revenue 6,806 8,055 Tax credit carryforwards 4,410 1,480 Capitalized start-up costs and other 907 193 Intangible Assets 1,071 1,150 Total deferred tax assets 41,899 20,686 Deferred tax liabilities: Depreciation and amortization (373 ) (277 ) Total deferred tax liabilities (373 ) (277 ) Valuation allowance (41,526 ) (20,409 ) Net deferred tax assets $ — $ —
Summary of Changes in the Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2016, 2015 and 2014, related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and development and orphan drug tax credit carryforwards and were as follows: Year Ended December 31, 2016 Year Ended December 31, 2015 Year Ended December 31, 2014 Valuation allowance at beginning of year $ (20,409 ) $ (6,120 ) $ (544 ) Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision (21,117 ) (14,289 ) (5,576 ) Valuation allowance at end of year $ (41,526 ) $ (20,409 ) $ (6,120 )

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2016 2015 2014 Numerator: Net loss $ (49,001 ) $ (35,056 ) $ (12,968 ) Accretion of convertible preferred stock to redemption value — (23 ) (71 ) Net loss attributable to common stockholders $ (49,001 ) $ (35,079 ) $ (13,039 ) Denominator: Weighted average common shares outstanding — basic and diluted 24,801,610 8,138,629 3,977,274 Less: Weighted average shares of unvested restricted common stock outstanding (114,277 ) (188,959 ) (366,682 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 24,915,887 7,949,670 3,610,592 Net loss per share attributable to common stockholders — basic and diluted $ (1.97 ) $ (4.41 ) $ (3.61 )
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an antidilutive effect: December 31, 2016 2015 2014 Convertible preferred stock (as converted to common stock) — — 3,421,142 Options to purchase common stock 4,185,699 3,097,022 1,104,172 Unvested restricted common stock 70,053 153,217 210,287 Warrant for the purchase of common stock — — 11,973 4,255,752 3,250,239 4,747,574

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments under Operating Leases	The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases: Year Ending December 31, 2017 1,205 2018 550 2019 505 2020 523 2021 132 Total $ 2,915

Quarterly Financial Data (Una36

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following tables present quarterly consolidated statement of operations data for fiscal 2016 and 2015. The below data is unaudited but, in our opinion, reflects all adjustments necessary for a fair presentation of this data in accordance with GAAP. Three Months Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (unaudited) Revenue $ 2,206 $ 2,371 $ 2,762 $ 4,132 Operating expenses 11,746 14,476 17,101 17,198 Net loss attributable to common stockholders $ (9,512 ) $ (12,099 ) $ (14,365 ) $ (13,027 ) Net loss per share attributable to common stockholders — basic and diluted $ (0.38 ) $ (0.49 ) $ (0.58 ) $ (0.52 ) Weighted average shares outstanding — basic and diluted 24,851,933 24,899,479 24,930,348 24,960,899 Three Months Ended March 31, 2015 June 30, 2015 September 30, 2015 December 31, 2015 (unaudited) Revenue $ 1,594 $ 1,742 $ 2,061 $ 2,352 Operating expenses 6,959 11,688 12,706 11,361 Net loss attributable to common stockholders $ (5,408 ) $ (9,976 ) $ (10,667 ) $ (9,028 ) Net loss per share attributable to common stockholders — basic and diluted $ (1.40 ) $ (2.57 ) $ (2.73 ) $ (0.45 ) Weighted average shares outstanding — basic and diluted 3,853,262 3,881,288 3,907,196 20,023,659

Nature of the Business and Ba37

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Accumulated deficit	$ 100,195	$ 51,194
Loss	49,001	35,056	$ 12,968
Cash used in operations	(47,644)	$ (32,029)	$ 8,760
Cash, cash equivalents and marketable securities	77,949
Expected Milestones	$ 15,000

Summary of Significant Accoun38

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs	$ 145	$ 0
Income tax examination, likelihood of settlement, description	The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement.
Income tax examination, likelihood of settlement, percentage	50.00%
Computer Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	5 years
Laboratory Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	5 years
Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	7 years
Leasehold Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	5 years
ATM [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs	$ 145

Fair Value Measurements and M39

Fair Value Measurements and Marketable Securities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Cash equivalents:
Cash equivalents	$ 30,234	$ 127,047
Marketable securities:
Marketable securities	47,715	0
Total cash equivalents and marketable securities	77,949
Money Market Funds [Member]
Cash equivalents:
Cash equivalents	22,988	127,047
U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents	7,246
Marketable securities:
Marketable securities	18,230
Commercial Paper [Member]
Marketable securities:
Marketable securities	15,814
Corporate Bonds [Member]
Marketable securities:
Marketable securities	13,671
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	22,988	127,047
Marketable securities:
Marketable securities	0
Total cash equivalents and marketable securities	22,988
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	22,988	127,047
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents	0
Marketable securities:
Marketable securities	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	7,246	0
Marketable securities:
Marketable securities	47,715
Total cash equivalents and marketable securities	54,961
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents	7,246
Marketable securities:
Marketable securities	18,230
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	15,814
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	13,671
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0	0
Marketable securities:
Marketable securities	0
Total cash equivalents and marketable securities	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	0	$ 0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents	0
Marketable securities:
Marketable securities	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	$ 0

Fair Value Measurements and M40

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument Fair Value Carrying Value [Abstract]
Assets, Level 1 to Level 2 transfers, amount	$ 0	$ 0
Liabilities, Level 1 to Level 2 transfers, amount	0	0
Assets, Level 2 to Level 1 transfers, amount	0	0
Liabilities, Level 2 to Level 1 transfers, amount	$ 0	$ 0

Fair Value Measurements and M41

Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 47,766
Gross Unrealized Gains	0
Gross Unrealized Loss	(51)
Fair Value	47,715	$ 0
Commercial Paper (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,814
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	15,814
U.S. Government Securities (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,002
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	7,002
U.S. Government Securities (Due After 1 Year Through 2 Years) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,256
Gross Unrealized Gains	0
Gross Unrealized Loss	(29)
Fair Value	11,227
Corporate Bonds (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,182
Gross Unrealized Gains	0
Gross Unrealized Loss	(10)
Fair Value	9,172
Corporate Bonds (Due After 1 Year Through 2 Years) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	4,512
Gross Unrealized Gains	0
Gross Unrealized Loss	(12)
Fair Value	$ 4,500

Prepaid Expenses and Other Cu42

Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	$ 5,484	$ 2,740
Prepaid Clinical, Manufacturing, and Scientific Costs [Member]
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	4,406	1,824
Prepaid Insurance [Member]
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	637	587
Other [Member]
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	$ 441	$ 329

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 10,934	$ 4,219
Less: Accumulated depreciation and amortization	(2,532)	(880)
Property and equipment, net	8,402	3,339
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 8,114	3,663
Property and equipment, useful life	5 years
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 251	165
Property and equipment, useful life	5 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 310	193
Property and equipment, useful life	7 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,259	$ 198
Property and equipment, useful life	5 years

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,652	$ 716	$ 164

Accrued Expenses and Other Cu45

Accrued Expenses and Other Current Liabilities - Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Payroll and payroll-related costs	$ 2,234	$ 2,000
External research and development services	4,409	1,389
Professional fees	433	254
Other	171	72
Total	$ 7,247	$ 3,715

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($)	Feb. 29, 2016	Aug. 21, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Assumptions, expected dividend rate			0.00%	0.00%	0.00%
Principal repaid during period			$ 679,000	$ 394,000
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Borrowings under line of credit				1,750,000
Debt issuance cost				$ 35,000
Accrue interest at an annual rate			5.50%
Periodic payment term of interest and principal			36 months
Credit facility, Interest rate description			Outstanding principal under the loan and security agreement accrue interest at an annual rate of 5.5%, which was determined on the date of each drawdown based on an annual rate of 2.25% plus the greater of (1) 3.25% or (2) the rate of interest per annum from time to time published in the money rates section of The Wall Street Journal as the prime rate then in effect.
Warrant to purchase shares		11,973
Common stock exercise price		$ 0.56
Warrant exercised option cashless				11,306
Assumptions, expected term		10 years
Assumptions, risk free interest rate		2.41%
Assumptions, volatility rate		70.93%
Assumptions, expected dividend rate		0.00%
Loan processing fee		$ 30,000
Warrants fair value		5,000
Line of credit facility, periodic payment, interest			$ 217,000	$ 88,000
First Amendment to Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Compensating percentage				105.00%
Compensating balance, amount			$ 6,844,000
Second Amendment to Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Maximum borrowings under line of credit	$ 7,000,000
Periodic payment term of interest			6 months
Borrowings under line of credit			$ 5,801,000
Debt issuance cost			$ 22,000
Accrue interest at an annual rate			3.00%
Periodic payment term of interest and principal			36 months
Accrue interest at annual rate below prime rate, to be determined on date of drawdown	0.50%
Percentage of interest accrue on additional final payment upon maturity or early termination	6.00%
Debt instrument final payment of accrues expenses			$ 348,000
Effective interest rate of the outstanding debt			6.20%
Equipment Line of Credit [Member] | Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Maximum borrowings under line of credit		$ 1,750,000
Periodic payment term of interest			6 months
Minimum [Member]
Line of Credit Facility [Line Items]
Assumptions, expected term			5 years 3 months 18 days	5 years 7 months 6 days	5 years 6 months
Minimum [Member] | Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Accrue interest at an annual rate			2.25%
Maximum [Member]
Line of Credit Facility [Line Items]
Assumptions, expected term			6 years	9 years	10 years
Maximum [Member] | Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Accrue interest at an annual rate			3.25%

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Notes payable	$ 6,478	$ 1,356
Less: current portion	(2,361)	(574)
Notes payable, net of current portion	4,117	782
Debt discount, net of accretion	(30)	(23)
Accretion related to final payment	82	0
Notes payable, net of discount, long term	$ 4,169	$ 759

Notes Payable - Estimated Futur

Notes Payable - Estimated Future Principal Payments Due (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Notes payable	$ 6,478	$ 1,356
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
2,017	2,379
2,018	2,123
2,019	1,838
2,020	138
Notes payable	$ 6,478

Collaboration and License Agr49

Collaboration and License Agreements - Additional Information (Detail)	1 Months Ended							12 Months Ended
	Dec. 31, 2015USD ($)	Aug. 31, 2015USD ($)	May 31, 2015USD ($)	Mar. 31, 2015USD ($)Disease_Indications	Sep. 30, 2014USD ($)	Jun. 30, 2014USD ($)	Oct. 31, 2013USD ($)Disease_Indications	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)Disease_Indications	Dec. 31, 2014USD ($)
REGENX 2013 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront license payment received					$ 150,000
Annual maintenance fees							$ 35,000
Number of new disease indications to be elected by company under option agreement | Disease_Indications							2
Period to obtain exclusive license							1 year
Additional extension period of license agreement					6 months
Milestones achieved								$ 0	$ 0
REGENXBIO, Inc [Member] | 2015 Option and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees				$ 50,000
Number of new disease indications to be elected by company under option agreement | Disease_Indications				4
Option fee payable upon exercise of each option				$ 1,000,000
Number of new disease indications to be elected by company under option agreement, exercised | Disease_Indications									3
Payment for option exercised, fee	$ 1,000,000	$ 1,000,000	$ 1,000,000
Option and license agreement termination, description								Any options not exercised will automatically terminate in 2019 (which may be extended for additional time for a fee). The Company may terminate the REGENX 2015 Option and License Agreement upon prior written notice. REGENX may terminate the agreement if the Company or its controlling affiliates become insolvent, if the Company is greater than a specified number of days late in paying money due under the REGENX 2015 Option and License Agreement, or, effective immediately, if the Company or its affiliates commence certain actions relating to the licensed patents. Either party may terminate the REGENX 2015 Option and License Agreement for a material breach that is not cured within a specified number of days.
Maximum [Member] | REGENX 2013 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestones payable							$ 1,650,000
Maximum [Member] | REGENXBIO, Inc [Member] | 2015 Option and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of various substantive milestones								$ 9,000,000
Bayer Healthcare LLC [Member] | Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront license payment received						$ 20,000,000
Research agreement period						54 months
Research agreement commencement date						2014-06
Research payments earned								8,292,000	$ 7,740,000	$ 3,265,000
Deferred revenue	$ 20,505,000							$ 17,326,000	$ 20,505,000
Bayer Healthcare LLC [Member] | Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future milestone payments receivable under development and commercialization activities						$ 232,000,000
University of Pennsylvania [Member] | 2015 Sponsored Research and Option Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research agreement expire date								Dec. 31, 2017
University of Pennsylvania [Member] | 2016 Research, Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payment accrued								$ 0
University of Pennsylvania [Member] | Maximum [Member] | 2015 Sponsored Research and Option Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments for allowable commitments extending beyond termination								2,762,000
University of Pennsylvania [Member] | Maximum [Member] | 2016 Research, Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense expected to be incurred per subfield on clinical development								$ 5,000,000

Collaboration and License Agr50

Collaboration and License Agreements - Schedule of Revenue from Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Revenue	$ 4,132	$ 2,762	$ 2,371	$ 2,206	$ 2,352	$ 2,061	$ 1,742	$ 1,594	$ 11,471	$ 7,750	$ 2,750
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Revenue:
Revenue									$ 11,471	$ 7,750	$ 2,750

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Oct. 31, 2013$ / sharesshares	Dec. 31, 2016USD ($)LicenseResearch$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2014USD ($)	Feb. 28, 2015$ / sharesshares
Related Party Transaction [Line Items]
Common stock, par value | $ / shares		$ 0.0001	$ 0.0001
Licensing fees paid in common stock | shares		25,043,506	25,008,227
General and administrative expense		$ 12,780,000	$ 8,723,000	$ 2,727,000
Amounts due to related party		$ 0	522,000
REGENX Biosciences, LLC [Member]
Related Party Transaction [Line Items]
Common stock, par value | $ / shares	$ 0.0002923
Licensing fees paid in common stock | shares	3,421
Directors and Management of REGENX [Member]
Related Party Transaction [Line Items]
Licensing fees paid in common stock | shares	2,379,242
University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Licensing fees paid in common stock | shares	1,038,474
Number of research agreements | Research		2
Cash payment to related party			5,137,000	0
Amounts due to related party			522,000
Beacon Bioventures Fund III Limited Partnership [Member]
Related Party Transaction [Line Items]
Common stock, par value | $ / shares	$ 0.0002923
Consulting services paid in common stock | shares	153,951
Issuance and vesting of restricted common stock for consulting services | shares	85,528
Beacon Bioventures Fund III Limited Partnership [Member] | Restricted Common Stock [Member]
Related Party Transaction [Line Items]
General and administrative expense				42,000
Beacon Bioventures Fund III Limited Partnership [Member] | Series A Preferred Stock
Related Party Transaction [Line Items]
Preferred stock, shares purchased | shares	5,000,000				10,000,000
Shares issued, price per share | $ / shares	$ 1				$ 1
REGENXBIO, Inc [Member]
Related Party Transaction [Line Items]
Number of license agreements | License		2
Cash payment to related party			3,890,000	7,341,000
Amounts due to related party			0
Fidelity Biosciences [Member]
Related Party Transaction [Line Items]
Cash payment to related party			1,000	$ 18,000
Amounts due to related party			$ 0

Related Party Transactions - Su

Related Party Transactions - Summary of Related Party Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Related Parties:
Related party expenses	$ 8,799	$ 7,184
REGENXBIO, Inc [Member]
Related Parties:
Related party expenses	3,140	7,139
University of Pennsylvania [Member]
Related Parties:
Related party expenses	$ 5,659
Fidelity Biosciences [Member]
Related Parties:
Related party expenses		$ 45

Stockholder's Equity - Addition

Stockholder's Equity - Additional Information (Detail)	Nov. 25, 2015USD ($)$ / sharesshares	Oct. 27, 2015USD ($)$ / sharesshares	Oct. 09, 2015	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2014USD ($)
Class of Stock [Line Items]
Reverse stock split conversion description				The Company effected a one-for-2.923 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company's Convertible Preferred Stock.
Reverse stock split conversion ratio			0.34211
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts | $				$ 0	$ 67,762,000	$ 0
Preferred stock, shares authorized				5,000,000	5,000,000
Preferred stock, par value | $ / shares				$ 0.0001	$ 0.0001
Common stock, shares authorized				150,000,000	150,000,000
Common stock, par value | $ / shares				$ 0.0001	$ 0.0001
Dividends | $				$ 0	$ 0
Description of common stock voting rights				Each share of common stock entitles the holder to one vote
Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued for initial public offering					5,604,775
Conversion of redeemable convertible preferred stock to common stock, shares					15,286,968
Common Stock [Member] | IPO [Member]
Class of Stock [Line Items]
Number of shares issued for initial public offering		5,500,000
Shares issued, price per share | $ / shares		$ 13
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts | $		$ 63,368,000
Number of common shares issued on conversion of convertible preferred stock		15,286,968
Conversion of redeemable convertible preferred stock to common stock, shares		15,286,968
Common Stock [Member] | Overallotment Option Granted to Underwriters [Member]
Class of Stock [Line Items]
Number of shares issued for initial public offering	104,775
Shares issued, price per share | $ / shares	$ 13
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts | $	$ 1,252,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 02, 2015	Apr. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future grant			1,223,561
Stock options granted in connection with employee stock purchase plan			1,192,814
Common stock reserved for issuance			5,889,101
Weighted average grant-date fair value of stock options granted			$ 7.15	$ 4.49
Unvested service-based stock options held by nonemployees			0	3,527
Total unrecognized compensation cost related to the unvested awards			$ 7,930
Unrecognized compensation cost related to the unvested awards, weighted-average period			2 years 7 months 6 days
Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted common stock vested			$ 362	$ 286
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term			5 years 3 months 18 days	5 years 7 months 6 days	5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term			6 years	9 years	10 years
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term			10 years
Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		102,086
2013 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future grant			0
2015 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance, percentage of number of shares of common stock outstanding			4.00%	4.00%
Additional common stock reserved for issuance				1,000,329
2015 Stock Option and Incentive Plan [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period			4 years
2015 Stock Option and Incentive Plan [Member] | Employees And Directors [Member] | Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan			0	0
2015 Stock Option and Incentive Plan [Member] | Employees And Directors [Member] | Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan			1,192,814	75,698
2015 Stock Option and Incentive Plan [Member] | Non Employees [Member] | Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan			0	0
2015 Stock Option and Incentive Plan [Member] | Non Employees [Member] | Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan			0	0
2015 Stock Option and Incentive Plan [Member] | Non Employees [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period			2 years
2015 Stock Option and Incentive Plan [Member] | Non Employees [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period			4 years
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan			0
Common stock reserved for issuance	256,585
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	1.00%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Assumptions Used in Fair Value of Options Granted to Employees and Directors Under Stock Option Plans (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, Minimum	83.00%	72.00%	71.00%
Expected volatility, Maximum	91.00%	87.00%	78.00%
Risk free interest rate, Minimum	1.00%	1.60%	1.70%
Risk free interest rate, Maximum	2.10%	2.40%	2.70%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	5 years 3 months 18 days	5 years 7 months 6 days	5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years	9 years	10 years

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Outstanding, Number of Shares, Beginning Balance	3,097,022
Granted, Number of Shares	1,192,814
Exercised, Number of Shares	(35,279)
Cancelled or forfeited, Number of Shares	(68,858)
Outstanding, Number of Shares, Ending Balance	4,185,699	3,097,022
Options vested and expected to vest, Number of Shares	4,185,699
Options exercisable, Number of Shares	1,553,567
Outstanding, Weighted Average Exercise Price, Beginning Balance	$ 3.11
Granted, Weighted Average Exercise Price	7.15
Exercised, Weighted Average Exercise Price	1.22
Cancelled or forfeited, Weighted Average Exercise Price	4.36
Outstanding, Weighted Average Exercise Price, Ending Balance	4.26	$ 3.11
Options vested and expected to vest, Weighted Average Exercise Price	4.26
Options exercisable, Weighted Average Exercise Price	$ 3.14
Outstanding, Weighted Average Remaining Contractual Term	8 years 4 months 24 days	9 years 2 months 12 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term	8 years 4 months 24 days
Options exercisable, Weighted Average Remaining Contractual Term	8 years 1 month 6 days
Outstanding, Aggregate Intrinsic Value	$ 4,757	$ 25,618
Options vested and expected to vest, Aggregate Intrinsic Value	4,757
Options exercisable, Aggregate Intrinsic Value	$ 2,644

Stock-Based Compensation - Su57

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Detail) - Restricted Common Stock [Member]	12 Months Ended
	Dec. 31, 2016$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Number of Shares, Beginning Balance | shares	153,217
Vested, Number of Shares | shares	(83,164)
Unvested restricted common stock, Number of Shares, Ending Balance | shares	70,053
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 0.87
Vested, Weighted Average Grant Date Fair Value | $ / shares	0.85
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 0.88

Stock-Based Compensation - Su58

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 3,367	$ 1,333	$ 86
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	1,147	664	46
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 2,220	$ 669	$ 40

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Income tax benefits	$ 0	$ 0
Changes in ownership interest of shareholders or public groups	50.00%
Changes in ownership interest rate period	3 years
Unrecognized tax benefits	$ 0	0	$ 0
Accrued interest or penalties related to uncertain tax positions	$ 0	$ 0
Tax Year 2013 [Member]
Income Taxes [Line Items]
Open tax year	2,013
Tax Year 2014 [Member]
Income Taxes [Line Items]
Open tax year	2,014
Tax Year 2015 [Member]
Income Taxes [Line Items]
Open tax year	2,015
Latest Tax Year [Member]
Income Taxes [Line Items]
Open tax year	2,016
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 73,099,000
Net operating loss carryforwards expiration year beginning	2,033
Federal [Member] | Research and Development [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 3,785,000
Tax credit carryforwards expiration year beginning	2,033
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 73,498,000
Net operating loss carryforwards expiration year beginning	2,033
State [Member] | Research and Development [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 625,000
Tax credit carryforwards expiration year beginning	2,029
Federal and State [Member]
Income Taxes [Line Items]
Additional net operating loss carryforwards attributable to stock option exercises	$ 74,000

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
State income taxes, net of federal benefit	(5.10%)	(5.10%)	(5.20%)
Tax credits	(6.00%)	(2.60%)	(4.10%)
Nondeductible expenses paid in common stock	2.00%	0.90%	0.30%
Change in deferred tax asset valuation allowance	43.10%	40.80%	43.00%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 28,705	$ 9,808
Deferred revenue	6,806	8,055
Tax credit carryforwards	4,410	1,480
Capitalized start-up costs and other	907	193
Intangible Assets	1,071	1,150
Total deferred tax assets	41,899	20,686
Deferred tax liabilities:
Depreciation and amortization	(373)	(277)
Total deferred tax liabilities	(373)	(277)
Valuation allowance	$ (41,526)	$ (20,409)	$ (6,120)	$ (544)

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Valuation allowance at beginning of year	$ (20,409)	$ (6,120)	$ (544)
Increases recorded to income tax provision	(21,117)	(14,289)	(5,576)
Valuation allowance at end of year	$ (41,526)	$ (20,409)	$ (6,120)

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss									$ (49,001)	$ (35,056)	$ (12,968)
Accretion of convertible preferred stock to redemption value									0	(23)	(71)
Net loss attributable to common stockholders	$ (13,027)	$ (14,365)	$ (12,099)	$ (9,512)	$ (9,028)	$ (10,667)	$ (9,976)	$ (5,408)	$ (49,001)	$ (35,079)	$ (13,039)
Denominator:
Weighted average common shares outstanding — basic and diluted									24,801,610	8,138,629	3,977,274
Less: Weighted average shares of unvested restricted common stock outstanding									(114,277)	(188,959)	(366,682)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted	24,960,899	24,930,348	24,899,479	24,851,933	20,023,659	3,907,196	3,881,288	3,853,262	24,915,887	7,949,670	3,610,592
Net loss per share attributable to common stockholders — basic and diluted	$ (0.52)	$ (0.58)	$ (0.49)	$ (0.38)	$ (0.45)	$ (2.73)	$ (2.57)	$ (1.40)	$ (1.97)	$ (4.41)	$ (3.61)

Net Loss per Share - Schedule64

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	4,255,752	3,250,239	4,747,574
Convertible Preferred Stock (as Converted to Common Stock) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	0	0	3,421,142
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	4,185,699	3,097,022	1,104,172
Unvested Restricted Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	70,053	153,217	210,287
Warrant for the Purchase of Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	0	0	11,973

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended	12 Months Ended
	Nov. 30, 2015USD ($)ft²$ / ft²	Jun. 30, 2015	Apr. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2016USD ($)ft²	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Nov. 30, 2016USD ($)	Oct. 31, 2014USD ($)	Mar. 31, 2014USD ($)
Commitments and Contingencies [Line Items]
Security deposits paid by company										$ 88,000
Equipment-collateralized irrevocable standby letter of credit				$ 92,000					$ 81,000
Security deposits returned to company			$ 88,000
Lease payments include base rent per month				$ 53,000
Increase in lease payments include base rent per month					$ 62,000
Operating leases, rent expense					$ 960,000	$ 734,000	$ 227,000
Eunice Kennedy Shriver National Institute of Child Health and Human Development [Member]
Commitments and Contingencies [Line Items]
Research and development agreement period		3 years
Noncancelable research funding payments minimum amount committed for first year								$ 244,000
Noncancelable research funding payments minimum amount committed for second year								300,000
Noncancelable research funding payments minimum amount committed for third year								300,000
Research funding potential aggregate amount								$ 844,000
Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease, area of property leased | ft²					15,000
Lease expiration date					2018-01
Laboratory and Office Facility [Member] | Woburn, MA Facility [Member] | ARE-MA Region No.20 LLC [Member]
Commitments and Contingencies [Line Items]
Operating lease, area of property leased | ft²	17,600
Operating lease, term	5 years
Operating lease, annual rent	$ 342,000
Operating lease, annual increase in rent per square foot | $ / ft²	1
Operating lease, tenant improvement allowances per rentable square foot | $ / ft²	35

Commitments and Contingencies66

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Operating Leases (Detail) - Cambridge and Woburn Operating Leases [Member] $ in Thousands	Dec. 31, 2016USD ($)
Operating Leased Assets [Line Items]
2,017	$ 1,205
2,018	550
2,019	505
2,020	523
2,021	132
Total	$ 2,915

Quarterly Financial Data (Una67

Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 4,132	$ 2,762	$ 2,371	$ 2,206	$ 2,352	$ 2,061	$ 1,742	$ 1,594	$ 11,471	$ 7,750	$ 2,750
Operating expenses	17,198	17,101	14,476	11,746	11,361	12,706	11,688	6,959	60,521	42,715	15,701
Net loss attributable to common stockholders	$ (13,027)	$ (14,365)	$ (12,099)	$ (9,512)	$ (9,028)	$ (10,667)	$ (9,976)	$ (5,408)	$ (49,001)	$ (35,079)	$ (13,039)
Net loss per share attributable to common stockholders — basic and diluted	$ (0.52)	$ (0.58)	$ (0.49)	$ (0.38)	$ (0.45)	$ (2.73)	$ (2.57)	$ (1.40)	$ (1.97)	$ (4.41)	$ (3.61)
Weighted average common shares outstanding — basic and diluted	24,960,899	24,930,348	24,899,479	24,851,933	20,023,659	3,907,196	3,881,288	3,853,262	24,915,887	7,949,670	3,610,592