Dimension Therapeutics (DMTX) 10-K 24 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Mar. 17, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
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	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,020,727
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float	$ 128,982,773

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 127,047	$ 17,913
Accounts receivable	143	1,974
Prepaid expenses and other current assets	2,740	404
Total current assets	129,930	20,291
Property and equipment, net	3,339	1,780
Other assets		62
Total assets	133,269	22,133
Current liabilities:
Accounts payable	1,555	675
Accrued expenses and other current liabilities	3,715	1,080
Amounts due to related parties	522	750
Deferred revenue	6,835	5,129
Notes payable	574	358
Total current liabilities	13,201	7,992
Deferred revenue, net of current portion	13,670	17,513
Notes payable, net of discount and current portion	759	1,166
Other liabilities	56
Total liabilities	$ 27,686	$ 26,671
Commitments and contingencies (Note 15)
Convertible preferred stock (Series A and B), $0.0001 par value; zero shares and 45,000,000 shares authorized as of December 31, 2015 and 2014, respectively; zero shares and 10,000,000 shares issued and outstanding as of December 31, 2015 and 2014, respectively; aggregate liquidation preference of $0 and $10,000 as of December 31, 2015 and 2014, respectively.		$ 9,653
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value; 5,000,000 and zero shares authorized at December 31, 2015 and 2014, respectively; zero shares issued or outstanding at December 31, 2015 and 2014, respectively	$ 0	0
Common stock, $0.0001 par value; 150,000,000 and 65,700,000 shares authorized as of December 31, 2015 and 2014 respectively; 25,008,227 and 4,056,818 shares issued and outstanding as of December 31, 2015 and 2014, respectively.	2	0
Additional paid-in capital	156,775	1,947
Accumulated deficit	(51,194)	(16,138)
Total stockholders’ equity (deficit)	105,583	(14,191)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 133,269	$ 22,133

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Convertible preferred stock (Series A and B), par value	$ 0.0001	$ 0.0001
Convertible preferred stock (Series A and B), shares authorized	0	45,000,000
Convertible preferred stock (Series A and B), shares issued	0	10,000,000
Convertible preferred stock (Series A and B), shares outstanding	0	10,000,000
Convertible preferred stock (Series A and B), aggregate liquidation preference	$ 0	$ 10,000
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	0
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	65,700,000
Common stock, shares issued	25,008,227	25,008,227
Common stock, shares outstanding	4,056,818	4,056,818

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenue		$ 7,750	$ 2,750
Operating expenses:
Research and development	$ 2,806	33,992	12,974
General and administrative	364	8,723	2,727
Total operating expenses	3,170	42,715	15,701
Loss from operations	(3,170)	(34,965)	(12,951)
Interest income (expense), net		(91)	(17)
Net loss and comprehensive loss	(3,170)	(35,056)	(12,968)
Accretion of convertible preferred stock to redemption value	(8)	(23)	(71)
Net loss attributable to common stockholders	$ (3,178)	$ (35,079)	$ (13,039)
Net loss per share attributable to common stockholders — basic and diluted	$ (2.71)	$ (4.41)	$ (3.61)
Weighted average common shares outstanding — basic and diluted	1,173,631	7,949,670	3,610,592

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]
Beginning balance at Dec. 31, 2013	$ (1,285)					$ 1,885	$ (3,170)
Beginning balance, shares at Dec. 31, 2013					3,715,245
Beginning balance at Dec. 31, 2013		$ 4,707
Beginning balance, shares at Dec. 31, 2013		5,000,000
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
Issuance and vesting of restricted common stock for consulting services, value	42					42
Issuance of restricted common stock, shares					341,573
Stock-based compensation expense	86					86
Accretion of convertible preferred stock to redemption value	(71)	$ 71				(71)
Net loss	(12,968)						(12,968)
Ending balance at Dec. 31, 2014	$ (14,191)					1,947	(16,138)
Ending balance, shares at Dec. 31, 2014	4,056,818				4,056,818
Ending balance at Dec. 31, 2014	$ 9,653	$ 9,653
Ending balance, shares at Dec. 31, 2014	10,000,000	10,000,000
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				12,572
Exercise of common stock warrants, shares					11,306
Stock-based compensation expense	$ 1,333					1,333
Accretion of convertible preferred stock to redemption value	(23)	$ 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	4,056,818				25,008,227
Ending balance, shares at Dec. 31, 2015	0

Consolidated Statements of Con6

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Series A Convertible Preferred Stock [Member]
Issuance of convertible preferred stock, issuance costs	$ 301	$ 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	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (3,170)	$ (35,056)	$ (12,968)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Licensing fees paid in common stock	1,800
Consulting services paid in common stock	89		42
Depreciation and amortization expense	0	716	164
Stock-based compensation expense	4	1,333	86
Non-cash interest expense		9	3
Changes in operating assets and liabilities:
Accounts receivable	(1)	1,831	(1,973)
Prepaid expenses and other current assets	(3)	(2,336)	(375)
Accounts payable	1,018	1,148	785
Accrued expenses and other current liabilities and amounts due to related parties	71	2,407	354
Deferred revenue		(2,137)	22,642
Other liabilities		56
Net cash (used in) provided by operating activities	(192)	(32,029)	8,760
Cash flows from investing activities:
Purchases of property and equipment		(2,543)	(1,667)
Lease deposits		62	(88)
Net cash used in investing activities		(2,481)	(1,755)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	4,699	79,219	4,875
Proceeds from exercise of common stock options		5
Proceeds from issuance of notes payable and common stock warrant, net of issuance costs		194	1,526
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts		67,762
Payments of initial public offering costs		(3,142)
Repayment of notes payable		(394)
Net cash provided by financing activities	4,699	143,644	6,401
Net increase in cash and cash equivalents	4,507	109,134	13,406
Cash and cash equivalents at beginning of period		17,913	4,507
Cash and cash equivalents at end of period	4,507	127,047	17,913
Supplemental disclosure of cash flow information:
Interest paid		82	14
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value	$ 8	23	71
Issuance of common stock warrant in connection with notes payable			5
Purchases of property and equipment included in accounts payable and accrued expenses		9	$ 277
Conversion of preferred stock to common stock upon initial public offering		$ 88,895

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2015
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 a gene therapy platform company focused on discovering and developing new therapeutic products for people living with rare diseases associated with the liver and caused by genetic mutations. 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. As of December 31, 2015 and December 31, 2014, the Company had an accumulated deficit of $51,194 and $16,138, respectively. The Company expects to continue to generate operating losses in the foreseeable future. On October 27, 2015 the Company completed an initial public offering (“IPO”) of its common stock, 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 automatically 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. The Company may seek additional funding through public or private financings, debt financing, collaboration agreements or government grants. In connection with preparing for the IPO, 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. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into additional collaboration arrangements or obtain government grants. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. 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, 2015
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 and cash equivalents and accounts receivable. The Company generally invests its cash equivalents in a money market fund held at one financial institution. 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, 2015 and 2014, 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 the 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 did not record any deferred offering costs as of December 31, 2015 or 2014. 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. As of December 31, 2015 and 2014, the Company's cash equivalents consisted of investments in a money market fund. 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 Certain assets of the Company are carried at fair value under GAAP. 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. The Company's cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of accounts receivable, accounts payable, accrued expenses and amounts due to related parties approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the Company's outstanding notes payable (see Note 7) approximates fair value, reflecting interest rates currently available to the Company. The fair value of the notes payable was determined based on a discounted cash flow model, which represents a Level 3 measurement. Debt Issuance Costs The Company has early 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 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. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements. 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, 2015, and 2014 and for the period from inception (June 20, 2013) to December 31, 2013. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern In April 2015, the FASB issued ASU No. 2015-03, Interest — Imputation of Interest Subtopic 835-30 In November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Subtopic 740) 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)

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities 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, 2015 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 127,047 $ — $ — $ 127,047 $ 127,047 $ — $ — $ 127,047 Fair Value Measurements as of December 31, 2014 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 17,836 $ — $ — $ 17,836 $ 17,836 $ — $ — $ 17,836 As of December 31, 2015 and December 31, 2014, the Company’s cash equivalents, which were invested in a money market fund, were valued based on Level 1 inputs. During the years ended December 31, 2015 and 2014, there were no transfers between Level 1, Level 2 and Level 3.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 Prepaid clinical, manufacturing, and scientific costs $ 1,824 $ 201 Prepaid insurance $ 587 $ 10 Other 329 193 $ 2,740 $ 404

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2015
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 2015 2014 Laboratory equipment 5 years $ 3,663 $ 1,854 Computer equipment 5 years 165 37 Furniture and fixtures 7 years 193 — Leasehold improvements * 198 53 4,219 1,944 Less: Accumulated depreciation and amortization (880 ) (164 ) $ 3,339 $ 1,780 * Shorter of asset life or lease term Depreciation and amortization expense was $716, $164 and $0 for the years ended December 31, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013, respectively. Substantially all of the Company’s laboratory equipment servers as collateral for the Company’s standby letter of credit pursuant to the agreement for the Company’s leased facilities (see Note 15).

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 Payroll and payroll-related costs $ 2,000 $ 469 External research and development services 1,389 362 Professional fees 254 159 Other 72 90 $ 3,715 $ 1,080

Notes Payable

Notes Payable	12 Months Ended
	Dec. 31, 2015
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. Through December 31, 2015 and 2014, the Company has borrowed $1,750 and $1,556, respectively, under the loan and security agreement and incurred zero and $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, 2015, the Company was required to maintain a compensating balance of $1,431 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 with SVB, which provided for additional advances under an equipment line of up to $7,000 (see note 18). During the years ended December 31, 2015 and 2014, the Company made scheduled principal repayments totaling $394 and $0, respectively. Borrowings under the loan and security agreement are collateralized by substantially all of the Company’s assets, except for its 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. During the years ended December 31, 2015 and 2014, the Company recognized $88 and $14, respectively, of interest expense related to the loan and security agreement with SVB. Accretion of debt discount recorded as additional interest expense was $9 and $3 for the years ended December 31, 2015 and 2014, respectively. As of December 31, 2015 and 2014, the unamortized debt discount was $23 and $32, respectively. Estimated future principal payments due under the loan and security agreement with SVB are as follows: Year Ended December 31, 2016 $ 583 2017 583 2018 190 Total $ 1,356

Collaboration Agreement

Collaboration Agreement	12 Months Ended
	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration Agreement	8. Collaboration Agreement 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 and a right of first notice for gene therapy treatments for hemophilia B. 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. 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, 2015, the Company had not earned any milestone or royalty payments. Revenue recognized under the collaboration agreement with Bayer consisted of the following: December 31, Period from Inception (June 20, 2013) to December 31, 2015 2014 2013 Revenue: Bayer Collaboration Agreement $ 7,750 $ 2,750 $ — $ 7,750 $ 2,750 $ — During the years ended December 31, 2015 and 2014, research payments earned by the Company under the collaboration agreement totaled $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, 2015 and 2014, deferred revenue related to the collaboration agreement with Bayer totaled $20,505 and $22,642, 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.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions	9. Related Party Transactions In October 2013, the Company entered into a license agreement (described below) 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 the Trustees of the University of Pennsylvania ("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. The Company determined that the fair value of its common stock was $0.53 per share at date of issuance and, as a result, recorded research and development expense of $1,799 during the period from inception (June 20, 2013) to December 31, 2013. 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 (see Note 10), 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. Related to the issuance of the 153,951 shares of common stock, the Company recorded general and administrative expense of $81 during the period from inception (June 20, 2013) to December 31, 2013. 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 and $8 for the year ended December 31, 2014 and during the period from inception (June 20, 2013) to December 31, 2013, respectively. 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 a result of these circumstances, REGENX, Penn and Beacon Bioventures are related parties to the Company. The Company entered into the following agreements with these related parties: 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, 2015 and 2014, 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. Penn Sponsored Research and Option Agreement In January 2015, the Company entered into a sponsored research and option agreement with Penn, which has been subsequently amended, regarding the conduct of research by its School of Medicine for a two-year term ending December 31, 2016 (the "Penn SRA") and with an estimated aggregate cost of $5,929 for the first year of the two-year term. In consideration for the Company's funding of the research under the Penn SRA, Penn granted the Company an option to obtain a worldwide, royalty-bearing license within the scope of the research and patentable rights thereunder. This agreement allows for termination upon notice, provided that the Company reimburses Penn for any of its allowable commitments made to others at that time, in an amount not to exceed $1,433. Beacon Bioventures Fidelity Biosciences has provided consulting services to the Company. Related party expenses consisted of the following: December 31, Period from Inception (June 20, 2013) to December 31, 2015 2014 2013 Related Parties: REGENX - R&D Expense $ 3,140 $ 7,139 $ 2,204 Penn - R&D Expense 5,659 — 546 Fidelity Biosciences - G&A Expense — 45 272 $ 8,799 $ 7,184 $ 3,022 Cash payments made to REGENXBIO, Inc. (“REGENX”) during the years ended December 31, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013 were $3,890, $7,341, and $0, respectively. Amounts due to REGENX by the Company were $0 and $750 as of December 31, 2015 and 2014, respectively. Cash payments made to the University of Pennsylvania during the years ended December 31, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013 were $5,137, $0 and $0, respectively. Amounts due to University of Pennsylvania by the Company were $522 and none as of December 31, 2015 and 2014, respectively. Cash payments made to Fidelity Biosciences during the years ended December 31, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013 were $1, $18 and $169, respectively. There were no amounts due to Fidelity Biosciences by the Company as of December 31, 2015 or 2014.

Convertible Preferred Stock and

Convertible Preferred Stock and Undesignated Preferred Stock	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Convertible Preferred Stock and Undesignated Preferred Stock	10. Convertible Preferred Stock and Undesignated Preferred Stock Convertible Preferred Stock The Company has issued Series A convertible preferred stock and Series B convertible preferred stock (collectively, the "Convertible Preferred Stock"). As of December 31, 2015 and 2014, the Company's certificate of incorporation, as amended and restated, authorized the Company to issue no shares and 45,000,000 shares, respectively, of $0.0001 par value convertible preferred stock. Given certain redemption features of the convertible preferred stock that are not solely within the control of the Company, the Convertible Preferred Stock is classified outside of stockholders' equity (deficit). Prior to the amendments described below, per the Company's SPA (see Note 9), the maximum amount of Series A preferred stock investors could buy in the initial closing was capped at 10,000,000 shares of Series A preferred stock. Under the SPA, the investors agreed to purchase up to 15,000,000 shares of Series A preferred stock at a price of $1.00 per share, in each of a second and a third tranche, upon the Company achieving specified milestones. The Company determined that the future tranche obligations do not meet the definition of a freestanding financial instrument because, while separately exercisable, they were not legally detachable. Further, the Company determined that the embedded future tranche obligations did not require bifurcation for accounting purposes as they are clearly and closely related to the economic characteristics and risks of the initial preferred shares and would not qualify as a derivative on a standalone basis. In October 2013, the Company issued 5,000,000 shares in the initial closing of Series A convertible preferred stock to Beacon Bioventures at a price of $1.00 per share for proceeds of $4,699, net of issuance costs of $301. In February 2014, the Company issued 5,000,000 shares of Series A convertible preferred stock to a second investor at a price of $1.00 per share for proceeds of $4,875, net of issuance costs of $125. Concurrent with this issuance, the SPA was amended to restrict the Company from selling additional equity to new investors until the collaboration agreement with Bayer was signed (see Note 8), to provide Beacon Bioventures with an option to purchase additional shares of Series A convertible preferred stock upon the non-occurrence of the second and third tranches (the "non-milestone closings"), and to provide the second investor with a participation right to purchase the same amount as Beacon Bioventures. In February 2015, the Company issued 14,500,000 shares of Series A convertible preferred stock, par value $0.0001 per share, at a per share price of $1.00 for proceeds of $14,480, net of issuance costs of $20. In February 2015, the Stock Purchase Agreement was further amended to remove the investors’ obligations to provide funding in the second and third tranches to the Company upon achieving specified milestones and to remove the investors’ options to purchase additional shares in non-milestone closings. In April 2015, the Company issued 20,183,824 shares of Series B convertible preferred stock, par value $0.0001 per share, at a per share price of $3.2204 for proceeds of $64,739, net of issuance costs of $261. During the year ended December 31, 2015, all convertible preferred shares issued and outstanding were converted to 15,286,968 shares of common stock as part of the initial public offering. Prior to the Company’s October 27, 2015, IPO, the holders of the Convertible Preferred Stock had the following rights and preferences: Voting Rights The holders of Convertible Preferred Stock are entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote. Each preferred stockholder is entitled to the number of votes equal to the number of shares of common stock into which each preferred share could convert on the record date for determination of stockholders entitled to vote. In addition, as of December 31, 2014, the holders of a majority in voting power of the Series A preferred stock were entitled to elect three members of the board of directors of the Company. Prior to the Company’s October 27, 2015 IPO, the holders of the Series A preferred stock and Series B preferred stock were entitled to elect two members and one member, respectively, of the board of directors of the Company. Dividends The holders of Convertible Preferred Stock are entitled to receive, on a pari passu basis, dividends in preference to any dividend on common stock at the rate of 8% per year of the Original Issue Price (as defined below). Dividends shall be payable when, as and if declared by the board of directors of the Company and shall not be cumulative. The Company may not declare, pay or set aside any dividends on shares of any other class or series of capital stock of the Company unless the holders of Convertible Preferred Stock then outstanding first receive, or simultaneously receive, any declared dividends on each outstanding share of Convertible Preferred Stock. Through December 31, 2015 and 2014, no dividends had been declared or paid by the Company. The Original Issue Price is $1.00 per share for Series A preferred stock and $3.2204 per share for Series B preferred stock, subject in each case to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization with respect to the Convertible Preferred Stock. Liquidation In the event of any voluntary or involuntary liquidation, dissolution or winding-up of the Company or Deemed Liquidation Event (as defined below), the holders of Convertible Preferred Stock then outstanding shall be entitled to be paid, on a pari passu basis, out of the assets of the Company available for distribution to stockholders, and before any payment shall be made to holders of common stock, an amount equal to the Original Issue Price per share, plus any declared but unpaid dividends thereon. If upon such event, the assets of the Company available for distribution are insufficient to permit payment in full to the holders of Convertible Preferred Stock, the proceeds will be ratably distributed among the holders of Convertible Preferred Stock in proportion to the respective amounts that they would have received if they were paid in full. After payments have been made in full to the holders of Convertible Preferred Stock, then, to the extent available, the holders of the common stock and Convertible Preferred Stock are entitled to participate in the distribution of the remaining assets available for distribution, pro rata based on the number of shares of common stock held by each such holder (on an as-converted to common basis). Unless a majority of the holders of the then outstanding shares of Convertible Preferred Stock elect otherwise, a Deemed Liquidation Event shall include a merger or consolidation (other than one in which stockholders of the Company own a majority by voting power of the outstanding shares of the surviving or acquiring corporation) or a sale, lease, transfer, exclusive license or other disposition of all or substantially all of the assets of the Company. There were no outstanding shares of Convertible Preferred Stock as of December 31, 2015. As of December 31, 2014 the liquidation preference of the outstanding shares of Convertible Preferred Stock $ 10,000. Redemption In conjunction with the closing of the Series B preferred stock financing in April 2015, the redemption rights of the Series A preferred stock were removed at that time. As a result of the removal of the redemption rights, as of April 20, 2015, the Company ceased the periodic recording of adjustments to accrete the carrying value of Series A preferred stock to its aggregate redemption value through October 30, 2018. Prior to April 20, 2015, the carrying value of the Series A preferred stock was being accreted to its aggregate redemption value through October 30, 2018, which had been the first required redemption date, using the effective interest method. Conversion Each share of Convertible Preferred Stock is convertible into common stock at the option of the stockholder at any time after the date of issuance. In addition, each share of Convertible Preferred Stock will be automatically converted into shares of common stock, at the applicable conversion ratio then in effect, upon the earlier of (i) a qualified public offering with gross proceeds of at least $60,000 and at a price of at least $14.1198 per share, subject to appropriate adjustment for any stock dividend, stock split, combination or other similar recapitalization, and (ii) a date and time, or occurrence of an event, specified by vote or written consent of a majority of the holders of the then outstanding shares of Convertible Preferred Stock. The conversion ratio of Convertible Preferred Stock is determined by dividing the Original Issue Price per share by the Conversion Price. The initial Conversion Price is $2.923 per share for Series A preferred stock and $9.4132292 per share for Series B preferred stock, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or recapitalization affecting the Convertible Preferred Stock. In addition, in the event that any holder of shares of Convertible Preferred Stock does not participate in a Qualified Financing (as defined below) by purchasing, in aggregate, such holders' pro rata amount, then each share of Convertible Preferred Stock held by such holder will automatically convert into 1/10th of one share of common stock. A Qualified Financing is defined as each issuance or sale of additional shares of common stock with gross proceeds of at least $30,000, unless a majority of the outstanding shares of the holders of the Convertible Preferred Stock elect that such transaction should not be treated as a Qualified Financing. Undesignated Preferred Stock On October 27, 2015, in connection with the closing of the IPO, the Company effected its 4 th The newly authorized preferred stock was classified under stockholders’ equity (deficit) as of December 31, 2015. As of December 31, 2015, there were no shares of undesignated preferred stock outstanding.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Common Stock	11. Common Stock 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. 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. As of December 31, 2015 and 2014, the Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 150,000,000 shares and 65,700,000, respectively, of $0.0001 par value common stock. As of December 31, 2014 the voting, dividend and liquidation rights of the holders of the Company's common stock are subject to and qualified by the rights, powers and preferences of the holders of the Convertible Preferred Stock set forth above. 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, 2015, no dividends have been declared.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	12. Stock-Based Compensation 2013 Stock Plan The Company's 2013 Stock Plan, as amended, (the "2013 Plan") provides for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2013 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The total number of shares of common stock that may be issued under the 2013 Plan was 1,847,416 shares as of December 31, 2014 of which 347,042 shares remained available for future grant at December 31, 2014. In April and June 2015, the Company effected an increase in the number of shares of common stock reserved for issuance under the 2013 Plan from 1,847,416 shares to 3,520,396 shares. In October 2015, the 2013 Plan was replaced and the remaining 54,510 shares available for future grant was transferred to the new plan. Vesting periods are determined at the discretion of the board of directors. 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, 2015, under the 2013 Plan, the Company granted to employees and directors 35,788 shares of restricted common stock and options to purchase 1,938,705 shares of common stock. During the year ended December 31, 2014, the Company granted to employees and directors 242,247 shares of restricted common stock and options to purchase 1,097,661 shares of common stock. During the year ended December 31, 2015, under the 2013 Plan, the Company granted to non-employees no shares of restricted common stock and options to purchase no shares of common stock. During the year ended December 31, 2014, the Company granted to non-employees 99,326 shares of restricted common stock and options to purchase 6,511 shares of common stock. 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. The total number of shares that may be issued under the 2015 Plan was 1,431,867, of which 1,347,188 shares remained available for future grant as of December 31, 2015. 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. During the year ended December 31, 2015, under the 2015 Plan, the Company granted to employees and directors no shares of restricted common stock and options to purchase 75,698 shares of common stock. 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. 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, 2015 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, 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, 2015 2014 Expected dividend yield — — Expected volatility 72 - 87% 71 - 78% Risk free interest rate 1.6 - 2.4% 1.7 - 2.7% Expected term 5.6 - 9.0 years 5.5 - 10 years There were no options granted during the period from inception (June 20, 2013) to December 31, 2013. 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, 2013: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2013 — — Granted 1,104,172 $ 0.53 Outstanding as of December 31, 2014 1,104,172 $ 0.53 9.7 $ 67 Granted 2,014,403 $ 4.49 Exercised (12,572 ) $ 0.38 Cancelled or forfeited (8,981 ) $ 0.38 Outstanding as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Options vested and expected to vest as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Options exercisable as of December 31, 2015 475,088 $ 1.35 8.9 $ 4,716 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, 2015. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2015 and 2014, was $4.49 and $0.53 per share, respectively. As of December 31, 2015 and 2014, there were outstanding unvested service-based stock options held by non-employees for the purchase of 3,527 and 5,155 shares of common stock, respectively. 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 inception (June 20, 2013) — $ — Issued 140,157 $ 0.53 Vested (2,276 ) $ 0.53 Unvested restricted common stock at December 31, 2013 137,881 $ 0.53 Issued 341,573 $ 0.53 Vested (269,167 ) $ 0.53 Unvested restricted common stock at December 31, 2014 210,287 $ 0.53 Issued 35,788 $ 3.57 Vested (92,858 ) $ 1.14 Unvested restricted common stock at December 31, 2015 153,217 $ 0.87 The total fair value of restricted common stock vested during the years ended December 31, 2015 and 2014, and the period from inception (June 20, 2013) to December 31, 2013 was $286, $157 and $1 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, Period from Inception (June 20, 2013) to December 31, 2015 2014 2013 Research and development $ 664 $ 46 $ 4 General and administrative 669 40 — $ 1,333 $ 86 $ 4 As of December 31, 2015, total unrecognized compensation cost related to the unvested awards was $5,537, which is expected to be recognized over a weighted average period of 3.3 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes During the years ended December 31, 2015 and 2014, 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, 2015 Year Ended December 31, 2014 Period from Inception (June 20, 2013) to December 31, 2013 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% State income taxes, net of federal benefit (5.1 ) (5.2 ) (2.1 ) Research and development tax credits (2.6 ) (4.1 ) (1.4 ) Nondeductible expenses paid in common stock 0.9 0.3 20.3 Change in deferred tax asset valuation allowance 40.8 43.0 17.2 Effective income tax rate 0.0 % 0.0 % 0.0 % Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Deferred tax assets: Net operating loss carryforwards $ 9,808 $ 4,377 Deferred revenue 8,055 1,131 Research and development tax credit carryforwards 1,480 572 Capitalized start-up costs and other 193 51 Intangible Assets 1,150 — Total deferred tax assets 20,686 6,131 Deferred tax liabilities: Depreciation and amortization (277 ) (11 ) Total deferred tax liabilities (277 ) (11 ) Valuation allowance (20,409 ) (6,120 ) Net deferred tax assets $ — $ — As of December 31, 2015, the Company had net operating loss carryforwards for federal and state income tax purposes of $25,118 and $24,014, 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, 2015, the Company also had available research and development tax credit carryforwards for federal and state income tax purposes of $1,210 and $290 respectively, which begin to expire in 2033 and 2029, respectively. Utilization of the net operating loss carryforwards and research and development 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 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 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 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, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013. 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, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013, related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and development tax credit carryforwards and were as follows: Year Ended December 31, 2015 Year Ended December 31, 2014 Year Ended December 31, 2013 Valuation allowance at beginning of year $ (6,120 ) $ (544 ) $ — Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision (14,289 ) (5,576 ) (544 ) Valuation allowance at end of year $ (20,409 ) $ (6,120 ) $ (544 ) The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2015, 2014 or 2013. 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, 2015
Earnings Per Share [Abstract]
Net Loss per Share	14. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, Period from Inception (June December 31, 2015 2014 2013 Numerator: Net loss $ (35,056 ) $ (12,968 ) $ (3,170 ) Accretion of convertible preferred stock to redemption value (23 ) (71 ) (8 ) Net loss attributable to common stockholders $ (35,079 ) $ (13,039 ) $ (3,178 ) Denominator: Weighted average common shares outstanding — basic and diluted 8,138,629 3,977,274 1,219,363 Less: Weighted average shares of unvested restricted common stock outstanding (188,959 ) (366,682 ) (45,732 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 7,949,670 3,610,592 1,173,631 Net loss per share attributable to common stockholders — basic and diluted $ (4.41 ) $ (3.61 ) $ (2.71 ) 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, 2015 2014 2013 Convertible preferred stock (as converted to common stock) — 3,421,142 1,710,571 Options to purchase common stock 3,097,022 1,104,172 — Unvested restricted common stock 153,217 210,287 137,881 Warrant for the purchase of common stock — 11,973 — 3,250,239 4,747,574 1,848,452

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	15. 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 will replace 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. Under the terms of the lease, the first monthly payment was paid in February 2016. The lease will commence upon substantial completion of building improvements, which is currently estimated to be on or around April 1, 2016 and will continue 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 will contribute $35.00 per rentable square foot towards a tenant improvement allowance, which is 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 $734, $227 and $0 during the years ended December 31, 2015 and 2014 and the period from inception (June 20, 2013) to December 31, 2013, respectively. The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases: Year Ending December 31, 2016 $ 1,055 2017 1,205 2018 549 2019 505 2020 523 2021 132 Total $ 3,969 The Company is further responsible for its pro rata share of operating expenses, real estate taxes and utilities of the facility housing the leased office and laboratory space. License Agreements The Company has entered into two license agreements with REGENX under which it is obligated to make contingent payments (see Note 9). 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, the Company is obligated to make noncancelable research funding payments of $244 annually, aggregating total payments of $732. 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, 2015.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2015
Postemployment Benefits [Abstract]
401(k) Savings Plan	16. 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, 2015
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	17. Quarterly Financial Data (Unaudited) The following tables present quarterly consolidated statement of operations data for fiscal 2015 and 2014. 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, 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 Three Months Ended March 31, 2014 June 30, 2014 September 30, 2014 December 31, 2014 (unaudited) Revenue $ — $ 159 $ 1,184 $ 1,407 Operating expenses 1,955 4,070 3,668 6,008 Net loss attributable to common stockholders $ (1,970 ) $ (3,929 ) $ (2,502 ) $ (4,636 ) Net loss per share attributable to common stockholders — basic and diluted $ (0.55 ) $ (1.10 ) $ (0.70 ) $ (1.25 ) Weighted average shares outstanding — basic and diluted 3,578,768 3,582,192 3,585,603 3,694,803

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	18. Subsequent Events On February 9, 2016, Dimension Therapeutics, Inc. (the “Company”) entered into a Second Amendment to the loan and security agreement (the “Second Amendment”) with Silicon Valley Bank. 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 Bank on the Funding Date of the applicable Equipment Advance and shall be payable monthly in arrears. In addition, upon the final monthly payment of each advance, the Company is require to make an additional payment equal to 6% of each advance original principal amount. Except as specifically amended by the Second Amendment, the Loan Agreement remains in full force and effect.

Summary of Significant Accoun26

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
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 and cash equivalents and accounts receivable. The Company generally invests its cash equivalents in a money market fund held at one financial institution. 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, 2015 and 2014, 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 the 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 did not record any deferred offering costs as of December 31, 2015 or 2014.
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. As of December 31, 2015 and 2014, the Company's cash equivalents consisted of investments in a money market fund.
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 Certain assets of the Company are carried at fair value under GAAP. 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. The Company's cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of accounts receivable, accounts payable, accrued expenses and amounts due to related parties approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the Company's outstanding notes payable (see Note 7) approximates fair value, reflecting interest rates currently available to the Company. The fair value of the notes payable was determined based on a discounted cash flow model, which represents a Level 3 measurement.
Debt Issuance Costs	Debt Issuance Costs The Company has early 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 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. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements.
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, 2015, and 2014 and for the period from inception (June 20, 2013) to December 31, 2013.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern In April 2015, the FASB issued ASU No. 2015-03, Interest — Imputation of Interest Subtopic 835-30 In November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Subtopic 740) 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)

Fair Value of Financial Asset27

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 127,047 $ — $ — $ 127,047 $ 127,047 $ — $ — $ 127,047 Fair Value Measurements as of December 31, 2014 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 17,836 $ — $ — $ 17,836 $ 17,836 $ — $ — $ 17,836

Prepaid Expenses and Other Cu28

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 Prepaid clinical, manufacturing, and scientific costs $ 1,824 $ 201 Prepaid insurance $ 587 $ 10 Other 329 193 $ 2,740 $ 404

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, net	Property and Equipment, net consisted of the following: December 31, Useful Life 2015 2014 Laboratory equipment 5 years $ 3,663 $ 1,854 Computer equipment 5 years 165 37 Furniture and fixtures 7 years 193 — Leasehold improvements * 198 53 4,219 1,944 Less: Accumulated depreciation and amortization (880 ) (164 ) $ 3,339 $ 1,780 * Shorter of asset life or lease term

Accrued Expenses and Other Cu30

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2015 2014 Payroll and payroll-related costs $ 2,000 $ 469 External research and development services 1,389 362 Professional fees 254 159 Other 72 90 $ 3,715 $ 1,080

Notes Payable (Tables)

Notes Payable (Tables)	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
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, 2016 $ 583 2017 583 2018 190 Total $ 1,356

Collaboration Agreement (Tables

Collaboration Agreement (Tables)	12 Months Ended
	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Revenue from Collaboration Agreement	Revenue recognized under the collaboration agreement with Bayer consisted of the following: December 31, Period from Inception (June 20, 2013) to December 31, 2015 2014 2013 Revenue: Bayer Collaboration Agreement $ 7,750 $ 2,750 $ — $ 7,750 $ 2,750 $ —

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Summary of Related Party Expenses	Related party expenses consisted of the following: December 31, Period from Inception (June 20, 2013) to December 31, 2015 2014 2013 Related Parties: REGENX - R&D Expense $ 3,140 $ 7,139 $ 2,204 Penn - R&D Expense 5,659 — 546 Fidelity Biosciences - G&A Expense — 45 272 $ 8,799 $ 7,184 $ 3,022

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
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, 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, 2015 2014 Expected dividend yield — — Expected volatility 72 - 87% 71 - 78% Risk free interest rate 1.6 - 2.4% 1.7 - 2.7% Expected term 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, 2013: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2013 — — Granted 1,104,172 $ 0.53 Outstanding as of December 31, 2014 1,104,172 $ 0.53 9.7 $ 67 Granted 2,014,403 $ 4.49 Exercised (12,572 ) $ 0.38 Cancelled or forfeited (8,981 ) $ 0.38 Outstanding as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Options vested and expected to vest as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Options exercisable as of December 31, 2015 475,088 $ 1.35 8.9 $ 4,716
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 inception (June 20, 2013) — $ — Issued 140,157 $ 0.53 Vested (2,276 ) $ 0.53 Unvested restricted common stock at December 31, 2013 137,881 $ 0.53 Issued 341,573 $ 0.53 Vested (269,167 ) $ 0.53 Unvested restricted common stock at December 31, 2014 210,287 $ 0.53 Issued 35,788 $ 3.57 Vested (92,858 ) $ 1.14 Unvested restricted common stock at December 31, 2015 153,217 $ 0.87
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, Period from Inception (June 20, 2013) to December 31, 2015 2014 2013 Research and development $ 664 $ 46 $ 4 General and administrative 669 40 — $ 1,333 $ 86 $ 4

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 Year Ended December 31, 2014 Period from Inception (June 20, 2013) to December 31, 2013 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% State income taxes, net of federal benefit (5.1 ) (5.2 ) (2.1 ) Research and development tax credits (2.6 ) (4.1 ) (1.4 ) Nondeductible expenses paid in common stock 0.9 0.3 20.3 Change in deferred tax asset valuation allowance 40.8 43.0 17.2 Effective income tax rate 0.0 % 0.0 % 0.0 %
Summary of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Deferred tax assets: Net operating loss carryforwards $ 9,808 $ 4,377 Deferred revenue 8,055 1,131 Research and development tax credit carryforwards 1,480 572 Capitalized start-up costs and other 193 51 Intangible Assets 1,150 — Total deferred tax assets 20,686 6,131 Deferred tax liabilities: Depreciation and amortization (277 ) (11 ) Total deferred tax liabilities (277 ) (11 ) Valuation allowance (20,409 ) (6,120 ) 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, 2015 and 2014 and for the period from inception (June 20, 2013) to December 31, 2013, related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and development tax credit carryforwards and were as follows: Year Ended December 31, 2015 Year Ended December 31, 2014 Year Ended December 31, 2013 Valuation allowance at beginning of year $ (6,120 ) $ (544 ) $ — Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision (14,289 ) (5,576 ) (544 ) Valuation allowance at end of year $ (20,409 ) $ (6,120 ) $ (544 )

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2015
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, Period from Inception (June December 31, 2015 2014 2013 Numerator: Net loss $ (35,056 ) $ (12,968 ) $ (3,170 ) Accretion of convertible preferred stock to redemption value (23 ) (71 ) (8 ) Net loss attributable to common stockholders $ (35,079 ) $ (13,039 ) $ (3,178 ) Denominator: Weighted average common shares outstanding — basic and diluted 8,138,629 3,977,274 1,219,363 Less: Weighted average shares of unvested restricted common stock outstanding (188,959 ) (366,682 ) (45,732 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 7,949,670 3,610,592 1,173,631 Net loss per share attributable to common stockholders — basic and diluted $ (4.41 ) $ (3.61 ) $ (2.71 )
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, 2015 2014 2013 Convertible preferred stock (as converted to common stock) — 3,421,142 1,710,571 Options to purchase common stock 3,097,022 1,104,172 — Unvested restricted common stock 153,217 210,287 137,881 Warrant for the purchase of common stock — 11,973 — 3,250,239 4,747,574 1,848,452

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2016 $ 1,055 2017 1,205 2018 549 2019 505 2020 523 2021 132 Total $ 3,969

Quarterly Financial Data (Una38

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following tables present quarterly consolidated statement of operations data for fiscal 2015 and 2014. 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, 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 Three Months Ended March 31, 2014 June 30, 2014 September 30, 2014 December 31, 2014 (unaudited) Revenue $ — $ 159 $ 1,184 $ 1,407 Operating expenses 1,955 4,070 3,668 6,008 Net loss attributable to common stockholders $ (1,970 ) $ (3,929 ) $ (2,502 ) $ (4,636 ) Net loss per share attributable to common stockholders — basic and diluted $ (0.55 ) $ (1.10 ) $ (0.70 ) $ (1.25 ) Weighted average shares outstanding — basic and diluted 3,578,768 3,582,192 3,585,603 3,694,803

Nature of the Business and Ba39

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	Nov. 25, 2015USD ($)$ / sharesshares	Oct. 27, 2015USD ($)$ / sharesshares	Oct. 09, 2015	Dec. 31, 2015USD ($)shares	Dec. 31, 2014USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Accumulated deficit | $				$ 51,194	$ 16,138
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts | $				$ 67,762
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
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares issued for initial public offering | shares				5,604,775
Common Stock [Member] | IPO [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares issued for initial public offering | shares		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
Number of common shares issued on conversion of convertible preferred stock | shares		15,286,968		15,286,968
Common Stock [Member] | Overallotment Option Granted to Underwriters [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares issued for initial public offering | shares	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

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Deferred offering costs	$ 0	$ 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%

Fair Value of Financial Asset41

Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets:
Cash equivalents	$ 127,047	$ 17,836
Assets total	127,047	17,836
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	127,047	17,836
Assets total	$ 127,047	$ 17,836

Fair Value of Financial Asset42

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
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
Assets, transfers into Level 3, amount	0	0
Liabilities, transfers into Level 3, amount	0	0
Assets, transfers out of Level 3, amount	0	0
Liabilities, transfers out of Level 3, amount	$ 0	$ 0

Prepaid Expenses and Other Cu43

Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	$ 2,740	$ 404
Prepaid Clinical, Manufacturing, and Scientific Costs [Member]
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	1,824	201
Prepaid Insurance [Member]
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	587	10
Other [Member]
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid expenses and other current assets	$ 329	$ 193

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 4,219	$ 1,944
Less: Accumulated depreciation and amortization	(880)	(164)
Property and equipment, net	3,339	1,780
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,663	1,854
Property and equipment, useful life	5 years
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 165	37
Property and equipment, useful life	5 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 193
Property and equipment, useful life	7 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 198	$ 53

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 0	$ 716	$ 164

Accrued Expenses and Other Cu46

Accrued Expenses and Other Current Liabilities - Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Payables And Accruals [Abstract]
Payroll and payroll-related costs	$ 2,000	$ 469
External research and development services	1,389	362
Professional fees	254	159
Other	72	90
Total	$ 3,715	$ 1,080

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail)	Feb. 29, 2016USD ($)	Aug. 21, 2014USD ($)Installment$ / sharesshares	Dec. 31, 2015USD ($)shares	Dec. 31, 2014USD ($)	Feb. 09, 2016USD ($)
Line of Credit Facility [Line Items]
Principal repaid during period			$ 394,000	$ 0
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, Monthly repayment period			36 monthly installments of interest and principal
Borrowings under line of credit			$ 1,750,000	1,556,000
Debt issuance cost			$ 0	35,000
Accrue interest at an annual rate			5.50%
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 | shares		11,973
Common stock exercise price | $ / shares		$ 0.56
Warrant exercised option cashless | shares			11,306
Number of installments | Installment		36
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			$ 88,000	14,000
Accretion of debt discount			9,000	3,000
Unamortized debt discount			$ 23,000	$ 32,000
First Amendment to Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Compensating Percentage			105.00%
Compensating balance, amount			$ 1,431,000
Second Amendment to Loan and Security Agreement [Member] | Subsequent Event
Line of Credit Facility [Line Items]
Maximum borrowings under line of credit					$ 7,000,000
Line of credit facility, additional borrowing	$ 7,000,000
Minimum [Member]
Line of Credit Facility [Line Items]
Assumptions, expected term			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			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%
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
Credit facility, Monthly repayment period			Six monthly, interest-only payments

Notes Payable - Estimated Futur

Notes Payable - Estimated Future Principal Payments Due (Detail) - Loan and Security Agreement [Member] $ in Thousands	Dec. 31, 2015USD ($)
Line of Credit Facility [Line Items]
2,016	$ 583
2,017	583
2,018	190
Total	$ 1,356

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - Bayer Healthcare LLC [Member] - Collaboration Agreement [Member] - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative 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		$ 7,740,000	$ 3,265,000
Deferred revenue		$ 20,505,000	$ 22,642,000
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future milestone payments receivable under development and commercialization activities	$ 232,000,000

Collaboration Agreement - Sched

Collaboration Agreement - Schedule of Revenue from Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended							12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Revenue	$ 2,352	$ 2,061	$ 1,742	$ 1,594	$ 1,407	$ 1,184	$ 159	$ 7,750	$ 2,750
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Revenue:
Revenue								$ 7,750	$ 2,750

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail)	1 Months Ended							6 Months Ended	12 Months Ended
	Dec. 31, 2015USD ($)$ / sharesshares	Aug. 31, 2015USD ($)	May. 31, 2015USD ($)	Mar. 31, 2015USD ($)Disease_Indications	Jan. 31, 2015USD ($)	Sep. 30, 2014USD ($)	Oct. 31, 2013USD ($)$ / sharesshares	Dec. 31, 2013USD ($)	Dec. 31, 2015USD ($)Disease_Indications$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares	Feb. 28, 2015$ / sharesshares
Related Party Transaction [Line Items]
Common stock, par value | $ / shares	$ 0.0001								$ 0.0001	$ 0.0001
Licensing fees paid in common stock, shares | shares	25,008,227								25,008,227	25,008,227
Research and development expense								$ 2,806,000	$ 33,992,000	$ 12,974,000
General and administrative expense								364,000	8,723,000	2,727,000
Milestones achieved									0	0
REGENX Biosciences, LLC [Member]
Related Party Transaction [Line Items]
Common stock, par value | $ / shares							$ 0.0002923
Licensing fees paid in common stock, shares | shares							3,421
Common stock fair value per share | $ / shares							$ 0.53
Research and development expense							$ 1,799,000
Directors and Management of REGENX [Member]
Related Party Transaction [Line Items]
Licensing fees paid in common stock, shares | shares							2,379,242
University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Licensing fees paid in common stock, shares | shares							1,038,474
Cash payment to related party									5,137,000	0
Amounts due to related party	$ 522,000								$ 522,000	0
University of Pennsylvania [Member] | Sponsored Research and Option Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense					$ 5,929,000
Research agreement term					2 years
Research agreement expiration date									Dec. 31, 2016
University of Pennsylvania [Member] | Maximum [Member] | Sponsored Research and Option Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement for allowable commitments made					$ 1,433,000
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 | 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								8,000		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
Beacon Bioventures Fund III Limited Partnership [Member] | Fully-Vested Common Stock [Member]
Related Party Transaction [Line Items]
General and administrative expense								$ 81,000
REGENX 2013 License Agreement [Member]
Related Party Transaction [Line Items]
Annual maintenance fees							$ 35,000
Additional extension period of license agreement						6 months
Extension fee paid by company for extension of option						$ 150,000
REGENX 2013 License Agreement [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Milestones payable							$ 1,650,000
REGENXBIO, Inc [Member]
Related Party Transaction [Line Items]
Cash payment to related party									$ 3,890,000	7,341,000
Amounts due to related party	0								$ 0	750,000
REGENXBIO, Inc [Member] | 2015 Option and License Agreement [Member]
Related Party Transaction [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.
REGENXBIO, Inc [Member] | Maximum [Member] | 2015 Option and License Agreement [Member]
Related Party Transaction [Line Items]
Milestone payments upon achievement of various substantive milestones									$ 9,000,000
Fidelity Biosciences [Member]
Related Party Transaction [Line Items]
Cash payment to related party									1,000	18,000
Amounts due to related party	$ 0								$ 0	$ 0

Related Party Transactions - Su

Related Party Transactions - Summary of Related Party Expenses (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Related Parties:
Related party expenses	$ 3,022	$ 8,799	$ 7,184
REGENXBIO, Inc [Member]
Related Parties:
Related party expenses	2,204	3,140	7,139
University of Pennsylvania [Member]
Related Parties:
Related party expenses	546	$ 5,659
Fidelity Biosciences [Member]
Related Parties:
Related party expenses	$ 272		$ 45

Convertible Preferred Stock a53

Convertible Preferred Stock and Undesignated Preferred Stock - Additional Information (Detail) - USD ($)	Oct. 27, 2015	Apr. 30, 2015	Feb. 28, 2015	Feb. 28, 2014	Oct. 31, 2013	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Feb. 02, 2015
Class of Stock [Line Items]
Convertible preferred stock authorized							0	45,000,000
Convertible preferred stock, par value							$ 0.0001	$ 0.0001
Convertible preferred stock issued							0	10,000,000
Proceeds from issuance of convertible preferred stock, gross						$ 4,699,000	$ 79,219,000	$ 4,875,000
Stock issuance costs for convertible preferred stock							$ 3,142,000
Common stock dividend rate							8.00%
Dividends							$ 0	$ 0
Outstanding shares of convertible preferred stock							0	10,000,000
Convertible preferred stock (Series A and B), aggregate liquidation preference							$ 0	$ 10,000,000
Gross proceeds of common stock							$ 30,000,000
Undesignated preferred stock, shares authorized	5,000,000						5,000,000	0
Undesignated preferred stock, par value	$ 0.0001						$ 0.0001	$ 0.0001
Preferred stock, shares outstanding							0	0
Qualified Public Offering [Member]
Class of Stock [Line Items]
Shares issued, price per share							$ 14.1198
Gross proceeds of public offering							$ 60,000,000
Common Stock [Member] | IPO [Member]
Class of Stock [Line Items]
Shares issued, price per share	$ 13
Stock issued during period, shares, conversion of convertible securities	15,286,968						15,286,968
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock, par value			$ 0.0001
Maximum number of preferred shares investors could buy in the initial closing per the SPA					10,000,000
Number of shares investor agreed to purchase under SPA					15,000,000
Purchase price per share of shares agreed to purchase under stock purchase agreement					$ 1
Convertible preferred stock issued			14,500,000	5,000,000
Shares issued, price per share			$ 1	$ 1	$ 1
Proceeds from issuance of convertible preferred stock, gross			$ 14,480,000	$ 4,875,000
Stock issuance costs for convertible preferred stock			$ 20,000	$ 125,000
Conversion of stock Initial conversion price							$ 2.923
Series A Convertible Preferred Stock [Member] | Beacon Bioventures Fund III Limited Partnership [Member]
Class of Stock [Line Items]
Convertible preferred stock issued					5,000,000
Shares issued, price per share					$ 1
Proceeds from issuance of convertible preferred stock, gross					$ 4,699,000
Stock issuance costs for convertible preferred stock					$ 301,000
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock issued		20,183,824
Shares issued, price per share		$ 3.2204							$ 3.2204
Proceeds from issuance of convertible preferred stock, gross		$ 64,739,000
Stock issuance costs for convertible preferred stock		$ 261,000
Conversion of stock Initial conversion price							$ 9.4132292

Common Stock - Additional Infor

Common Stock - Additional Information (Detail)	Nov. 25, 2015USD ($)$ / sharesshares	Oct. 27, 2015USD ($)$ / sharesshares	Oct. 09, 2015	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares
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 | $				$ 67,762,000
Common stock, shares authorized				150,000,000	65,700,000
Common stock, par value | $ / shares				$ 0.0001	$ 0.0001
Dividends | $				$ 0	$ 0
Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued for initial public offering				5,604,775
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		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, 2013	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Oct. 31, 2015	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan			0		2,014,403	1,104,172
Weighted average grant-date fair value of stock options granted					$ 4.49	$ 0.53
Unvested service-based stock options held by nonemployees					3,527	5,155
Total unrecognized compensation cost related to the unvested awards					$ 5,537
Unrecognized compensation cost related to the unvested awards, weighted-average period					3 years 3 months 18 days
Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan				140,157	35,788	341,573
Shares granted in connection with employee stock purchase plan				140,157	35,788	341,573
Total fair value of restricted common stock vested				$ 1	$ 286	$ 157
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions, expected term					5 years 7 months 6 days	5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions, expected term					9 years	10 years
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions, expected term					10 years
Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan		102,086
2013 Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of estimated fair value of the shares on the date of grant					100.00%
Stock options expiration period					10 years
Common stock reserved for issuance						1,847,416		3,520,396
Common stock reserved for future grant						347,042	54,510
Shares granted in connection with employee stock purchase plan					0
2013 Stock Plan [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period					4 years
2013 Stock Plan [Member] | Employees And Directors [Member] | Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan					35,788	242,247
Shares granted in connection with employee stock purchase plan					35,788	242,247
2013 Stock Plan [Member] | Employees And Directors [Member] | Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan					1,938,705	1,097,661
2013 Stock Plan [Member] | Non Employees [Member] | Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan					0	99,326
Shares granted in connection with employee stock purchase plan					0	99,326
2013 Stock Plan [Member] | Non Employees [Member] | Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan					0	6,511
Shares granted in connection with employee stock purchase plan					0	6,511
2013 Stock Plan [Member] | Non Employees [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period					2 years
2013 Stock 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 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance					1,431,867
Common stock reserved for future grant					1,347,188
Common stock reserved for issuance, percentage of number of shares of common stock outstanding					4.00%
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]
Shares granted in connection with employee stock purchase plan					0
Shares granted in connection with employee stock purchase plan					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]
Shares granted in connection with employee stock purchase plan					75,698
Shares granted in connection with employee stock purchase plan					75,698
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	256,585
Shares granted in connection with employee stock purchase plan					0
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, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	72.00%	71.00%
Expected volatility, Maximum	87.00%	78.00%
Risk free interest rate, Minimum	1.60%	1.70%
Risk free interest rate, Maximum	2.40%	2.70%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	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	9 years	10 years

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Outstanding, Number of Shares, Beginning Balance		1,104,172
Granted, Number of Shares	0	2,014,403	1,104,172
Outstanding, Number of Shares, Ending Balance		3,097,022	1,104,172
Exercised, Number of Shares		(12,572)
Cancelled or forfeited, Number of Shares		(8,981)
Options vested and expected to vest, Number of Shares		3,097,022
Options exercisable, Number of Shares		475,088
Outstanding, Weighted Average Exercise Price, Beginning Balance		$ 0.53
Granted, Weighted Average Exercise Price		4.49	$ 0.53
Outstanding, Weighted Average Exercise Price, Ending Balance		3.11	$ 0.53
Exercised, Weighted Average Exercise Price		0.38
Cancelled or forfeited, Weighted Average Exercise Price		0.38
Options vested and expected to vest, Weighted Average Exercise Price		3.11
Options exercisable, Weighted Average Exercise Price		$ 1.35
Outstanding, Weighted Average Remaining Contractual Term		9 years 2 months 12 days	9 years 8 months 12 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term		9 years 2 months 12 days
Options exercisable, Weighted Average Remaining Contractual Term		8 years 10 months 24 days
Outstanding, Aggregate Intrinsic Value		$ 25,618	$ 67
Options vested and expected to vest, Aggregate Intrinsic Value		25,618
Options exercisable, Aggregate Intrinsic Value		$ 4,716

Stock-Based Compensation - Su58

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Detail) - Restricted Common Stock [Member] - $ / shares	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Number of Shares, Beginning Balance		210,287	137,881
Shares granted in connection with employee stock purchase plan	140,157	35,788	341,573
Vested, Number of Shares	(2,276)	(92,858)	(269,167)
Unvested restricted common stock, Number of Shares, Ending Balance	137,881	153,217	210,287
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Beginning Balance		$ 0.53	$ 0.53
Issued, Weighted Average Grant Date Fair Value	$ 0.53	3.57	0.53
Vested, Weighted Average Grant Date Fair Value	0.53	1.14	0.53
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Ending Balance	$ 0.53	$ 0.87	$ 0.53

Stock-Based Compensation - Su59

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 4	$ 1,333	$ 86
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 4	664	46
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		$ 669	$ 40

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 25,118,000
Net operating loss carryforwards expiration year beginning	2,033
Federal [Member] | Research and Development [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 1,210,000
Tax credit carryforwards expiration year beginning	2,033
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 24,014,000
Net operating loss carryforwards expiration year beginning	2,033
State [Member] | Research and Development [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 290,000
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)	6 Months Ended	12 Months Ended
	Dec. 31, 2013	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	(2.10%)	(5.10%)	(5.20%)
Research and development tax credits	(1.40%)	(2.60%)	(4.10%)
Nondeductible expenses paid in common stock	20.30%	0.90%	0.30%
Change in deferred tax asset valuation allowance	17.20%	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, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 9,808	$ 4,377
Deferred revenue	8,055	1,131
Research and development tax credit carryforwards	1,480	572
Capitalized start-up costs and other	193	51
Intangible Assets	1,150
Total deferred tax assets	20,686	6,131
Deferred tax liabilities:
Depreciation and amortization	(277)	(11)
Total deferred tax liabilities	(277)	(11)
Valuation allowance	$ (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, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Valuation allowance at beginning of year	$ (6,120)	$ (544)
Increases recorded to income tax provision	(14,289)	(5,576)	$ (544)
Valuation allowance at end of year	$ (20,409)	$ (6,120)	$ (544)

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								6 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss									$ (3,170)	$ (35,056)	$ (12,968)
Accretion of convertible preferred stock to redemption value									(8)	(23)	(71)
Net loss attributable to common stockholders	$ (9,028)	$ (10,667)	$ (9,976)	$ (5,408)	$ (4,636)	$ (2,502)	$ (3,929)	$ (1,970)	$ (3,178)	$ (35,079)	$ (13,039)
Denominator:
Weighted average common shares outstanding — basic and diluted									1,219,363	8,138,629	3,977,274
Less: Weighted average shares of unvested restricted common stock outstanding									(45,732)	(188,959)	(366,682)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted	20,023,659	3,907,196	3,881,288	3,853,262	3,694,803	3,585,603	3,582,192	3,578,768	1,173,631	7,949,670	3,610,592
Net loss per share attributable to common stockholders — basic and diluted	$ (0.45)	$ (2.73)	$ (2.57)	$ (1.40)	$ (1.25)	$ (0.70)	$ (1.10)	$ (0.55)	$ (2.71)	$ (4.41)	$ (3.61)

Net Loss per Share - Schedule65

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	3,250,239	4,747,574	1,848,452
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		3,421,142	1,710,571
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	3,097,022	1,104,172
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	153,217	210,287	137,881
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		11,973

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Nov. 30, 2015USD ($)ft²$ / ft²	Jun. 30, 2015USD ($)	Apr. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2015USD ($)ft²	Dec. 31, 2015USD ($)ft²	Dec. 31, 2014USD ($)	Oct. 31, 2014USD ($)	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					$ 0		$ 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		$ 244,000
Research funding potential aggregate amount		$ 732,000
Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease, area of property leased | ft²						15,000	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
Operating lease, annual increase in rent per square foot | $ / ft²	1
Operating lease, tenant improvement allowance per rentable square foot | $ / ft²	35

Commitments and Contingencies67

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Operating Leases (Detail) - Cambridge and Woburn Operating Leases [Member] $ in Thousands	Dec. 31, 2015USD ($)
Operating Leased Assets [Line Items]
2,016	$ 1,055
2,017	1,205
2,018	549
2,019	505
2,020	523
2,021	132
Total	$ 3,969

Quarterly Financial Data (Una68

Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 2,352	$ 2,061	$ 1,742	$ 1,594	$ 1,407	$ 1,184	$ 159			$ 7,750	$ 2,750
Operating expenses	11,361	12,706	11,688	6,959	6,008	3,668	4,070	$ 1,955	$ 3,170	42,715	15,701
Net loss attributable to common stockholders	$ (9,028)	$ (10,667)	$ (9,976)	$ (5,408)	$ (4,636)	$ (2,502)	$ (3,929)	$ (1,970)	$ (3,178)	$ (35,079)	$ (13,039)
Net loss per share attributable to common stockholders — basic and diluted	$ (0.45)	$ (2.73)	$ (2.57)	$ (1.40)	$ (1.25)	$ (0.70)	$ (1.10)	$ (0.55)	$ (2.71)	$ (4.41)	$ (3.61)
Weighted average common shares outstanding — basic and diluted	20,023,659	3,907,196	3,881,288	3,853,262	3,694,803	3,585,603	3,582,192	3,578,768	1,173,631	7,949,670	3,610,592

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event - Second Amendment to Loan and Security Agreement [Member]	Feb. 09, 2016USD ($)
Subsequent Event [Line Items]
Maximum borrowings under line of credit	$ 7,000,000
Periodic payment term of interest	6 months
Periodic payment term of interest and principal	36 months
Annual accrue interest rate basis	Annual rate of 0.5% below the Prime Rate
Percentage of interest accrue on additional final payment upon maturity or early termination	6.00%

Uncategorized Items - dmtx-2015

Label	Element	Value
Stock Issued During Period Value Vesting Of Restricted Common Stock For Consulting Services	dmtx_StockIssuedDuringPeriodValueVestingOfRestrictedCommonStockForConsultingServices	$ 8,000
Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value	us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4,000
Stock Issued During Period Value Issued For Services	us-gaap_StockIssuedDuringPeriodValueIssuedForServices	81,000
Licensing Fees Paid In Common Stock Value	dmtx_LicensingFeesPaidInCommonStockValue	1,800,000
Temporary Equity Accretion To Redemption Value	us-gaap_TemporaryEquityAccretionToRedemptionValue	(8,000)
Additional Paid In Capital [Member]
Stock Issued During Period Value Vesting Of Restricted Common Stock For Consulting Services	dmtx_StockIssuedDuringPeriodValueVestingOfRestrictedCommonStockForConsultingServices	8,000
Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value	us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4,000
Stock Issued During Period Value Issued For Services	us-gaap_StockIssuedDuringPeriodValueIssuedForServices	81,000
Licensing Fees Paid In Common Stock Value	dmtx_LicensingFeesPaidInCommonStockValue	1,800,000
Temporary Equity Accretion To Redemption Value	us-gaap_TemporaryEquityAccretionToRedemptionValue	$ (8,000)
Common Stock [Member]
Stock Issued During Period Shares Issued For Services	us-gaap_StockIssuedDuringPeriodSharesIssuedForServices	153,951
Stock Issued During Period Shares Vesting Of Restricted Common Stock For Consulting Services	dmtx_StockIssuedDuringPeriodSharesVestingOfRestrictedCommonStockForConsultingServices	85,528
Stock Issued During Period Shares Restricted Stock Award Gross	us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross	54,629
Licensing Fees Paid In Common Stock Shares	dmtx_LicensingFeesPaidInCommonStockShares	3,421,137
Convertible Preferred Stock [Member]
Temporary Equity Accretion To Redemption Value	us-gaap_TemporaryEquityAccretionToRedemptionValue	$ 8,000
Retained Earnings [Member]
Net Income Loss	us-gaap_NetIncomeLoss	(3,170,000)
Series A Convertible Preferred Stock [Member] | Convertible Preferred Stock [Member]
Temporary Equity Stock Issued During Period Value New Issues	us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues	$ 4,699,000
Temporary Equity Stock Issued During Period Shares New Issues	dmtx_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues	5,000,000