Nightstar Therapeutics (NITE) 20-F 3 Apr 18 2017 FY Annual report (foreign) Financial data

Document and Entity Information

Document and Entity Information	12 Months Ended
	Dec. 31, 2017shares
Document And Entity Information [Abstract]
Document Type	20-F
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	NITE
Entity Registrant Name	Nightstar Therapeutics plc
Entity Central Index Key	1,711,675
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding	28,904,714

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 129,404	$ 10,122
Prepaid expenses and other assets	5,438	4,110
Total current assets	134,842	14,232
Property and equipment, net	355	363
Total assets	135,197	14,595
Current liabilities:
Accounts payable	3,196	1,229
Accrued expenses and other liabilities	6,189	4,322
Total current liabilities	9,385	5,551
Total liabilities	9,385	5,551
Commitments and contingencies (see Note 11)
Shareholders’ equity:
Ordinary shares, £0.01 nominal value; 100,000,000 and 70,000,000 shares authorized as of December 31, 2017 and 2016, respectively; 28,904,714 and 12,545,314 shares issued and outstanding at December 31, 2017 and 2016, respectively	1
Additional paid-in capital	185,943	43,478
Accumulated other comprehensive income (loss)	1,890	(2,098)
Accumulated deficit	(62,022)	(32,336)
Total shareholders’ equity	125,812	9,044
Total liabilities and shareholders’ equity	$ 135,197	$ 14,595

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - £ / shares	Dec. 31, 2017	Sep. 12, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Ordinary shares, nominal value	£ 0.01	£ 0.01	£ 0.01
Ordinary shares, authorized	100,000,000		70,000,000
Ordinary shares, issued	28,904,714		12,545,314	8,693,625
Ordinary shares, outstanding	28,904,714		12,545,314	8,693,625

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 20,502,000	$ 10,165,000	$ 7,175,000
General and administrative	7,001,000	2,055,000	2,149,000
Total operating expenses	27,503,000	12,220,000	9,324,000
Other income (expense):
Interest and other income	709,000	22,000	9,000
Other expense, net	(2,855,000)
Change in fair value of tranche obligations			(4,333,000)
Total other income (expense), net	(2,146,000)	22,000	(4,324,000)
Loss before provision for income taxes	(29,649,000)	(12,198,000)	(13,648,000)
Provision for income taxes	37,000	0	0
Net loss	(29,686,000)	(12,198,000)	(13,648,000)
Other comprehensive income (loss):
Foreign exchange translation adjustment	3,988,000	(1,385,000)	(541,000)
Total comprehensive loss	$ (25,698,000)	$ (13,583,000)	$ (14,189,000)
Basic and diluted net loss per ordinary share	$ (1.63)	$ (1.36)	$ (2.46)
Weighted average basic and diluted ordinary shares	18,186,752	8,954,388	5,539,374

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance at Dec. 31, 2014	$ 3,004		$ 9,666	$ (172)	$ (6,490)
Balance, shares at Dec. 31, 2014		2,085,962
Issuance of ordinary shares	13,959		13,959
Issuance of ordinary shares, shares		6,225,139
Issuance of restricted share awards, shares		382,524
Share-based compensation expense	112		112
Foreign exchange translation adjustment	(541)			(541)
Reclassification of tranche obligation liability to additional paid-in capital	8,216		8,216
Net loss	(13,648)				(13,648)
Balance at Dec. 31, 2015	11,102		31,953	(713)	(20,138)
Balance, shares at Dec. 31, 2015		8,693,625
Issuance of ordinary shares	11,317		11,317
Issuance of ordinary shares, shares		3,335,911
Issuance of restricted share awards, shares		515,778
Share-based compensation expense	208		208
Foreign exchange translation adjustment	(1,385)			(1,385)
Net loss	(12,198)				(12,198)
Balance at Dec. 31, 2016	9,044		43,478	(2,098)	(32,336)
Balance, shares at Dec. 31, 2016		12,545,314
Issuance of ordinary shares	141,093	$ 1	141,092
Issuance of ordinary shares, shares		15,898,420
Issuance of restricted share awards, shares		460,980
Share-based compensation expense	1,373		1,373
Foreign exchange translation adjustment	3,988			3,988
Net loss	(29,686)				(29,686)
Balance at Dec. 31, 2017	$ 125,812	$ 1	$ 185,943	$ 1,890	$ (62,022)
Balance, shares at Dec. 31, 2017		28,904,714

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (29,686,000)	$ (12,198,000)	$ (13,648,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	206,000	138,000	40,000
Non-cash share-based compensation	1,373,000	208,000	112,000
Change in fair value of tranche obligation liability			4,333,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(908,000)	(1,643,000)	(2,522,000)
Accounts payable	1,773,000	419,000	384,000
Accrued expenses and other liabilities	1,446,000	2,959,000	1,596,000
Net cash used in operating activities	(25,796,000)	(10,117,000)	(9,705,000)
Cash flows from investing activities:
Purchases of property and equipment	(166,000)	(306,000)	(289,000)
Net cash used in investing activities	(166,000)	(306,000)	(289,000)
Cash flows from financing activities:
Proceeds of issuance of ordinary shares, net of issuance costs	141,093,000	11,317,000	13,959,000
Net cash provided by financing activities	141,093,000	11,317,000	13,959,000
Effect of exchange rate changes on cash and cash equivalents	4,151,000	(1,448,000)	(451,000)
Net increase (decrease) in cash and cash equivalents	119,282,000	(554,000)	3,514,000
Cash and cash equivalents, beginning of period	10,122,000	10,676,000	7,162,000
Cash and cash equivalents, end of period	$ 129,404,000	$ 10,122,000	10,676,000
Supplemental non-cash activities:
Reclassification of tranche obligation liability to additional paid-in capital			$ 8,216,000

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Nightstar Therapeutics plc (the “Company”) is a clinical-stage gene therapy company focused on developing and commercializing novel one-time treatments for patients suffering from rare inherited retinal diseases that would otherwise progress to blindness. The Company is developing a pipeline of proprietary product candidates that are designed to substantially modify or halt the progression of inherited retinal diseases for which there are no currently approved treatments. The Company’s lead product candidate, NSR-REP1, for the treatment of choroideremia (“CHM”), is in Phase 3 clinical development. The Company is also conducting a Phase 1/2 clinical trial with its second product candidate, NSR-RPGR, for the treatment of X-linked retinitis pigmentosa. The Company also has product candidates in preclinical development for a number of inherited retinal diseases for which there are no approved treatments such as Stargardt disease and Best vitelliform macular dystrophy. The Company is a public limited company incorporated in England and Wales. On September 11, 2017, the Company completed the first step of a corporate reorganization, pursuant to which the shareholders of NightstaRx Limited, a private company originally incorporated under the laws of England and Wales in May 2013 as NewIncCo 1242 Limited which subsequently changed its name to NightstaRx Limited in January 2014, exchanged each of the different classes of ordinary shares held by them for the same number and class of newly issued ordinary shares of Nightstar Therapeutics Limited. As a result, NightstaRx Limited became a wholly owned subsidiary of Nightstar Therapeutics Limited, a holding company incorporated on July 6, 2017 with nominal assets and liabilities, which has not conducted any operations prior to the aforementioned share exchange and other actions incidental to the exchange and its incorporation. On September 15, 2017, as the second step of the corporate reorganization, Nightstar Therapeutics Limited re-registered as a public limited company and its name was changed from Nightstar Therapeutics Limited to Nightstar Therapeutics plc. On October 2, 2017, the different classes of ordinary shares were converted into a single class of ordinary shares and the Company completed its initial public offering (IPO) of American Depositary Shares (“ADS”). In the IPO, the Company sold an aggregate of 6,164,000 at a public offering price of $14.00 per ADS. Net proceeds were approximately $77.4 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company. The Company’s corporate reorganization is described further below. Nightstar Therapeutics plc is a continuation of NightstaRx Limited and its subsidiaries, and the corporate reorganization has been accounted for as a combination of entities under common control. The corporate reorganization has been given retrospective effect in these financial statements and such financial statements represent the financial statements of Nightstar Therapeutics plc. In connection with the corporate reorganization, outstanding restricted share awards of NightstaRx Limited were exchanged for share awards of Nightstar Therapeutics plc with identical restrictions. The accompanying consolidated financial statements include those of Nightstar Therapeutics plc, NightstaRx Limited and its U.S. subsidiary, Nightstar, Inc., which was formed in 2016. For purposes of the disclosures below, unless otherwise indicated or the context otherwise requires, references to the “Company” relate to the consolidated financial statements of NightstaRx Limited and its wholly owned U.S. subsidiary, Nightstar, Inc., prior to the corporate reorganization and the newly formed Nightstar Therapeutics plc and its subsidiaries after the completion of the corporate reorganization. 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 the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of 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 revenue from its product sales. The Company has funded its operations primarily with proceeds from the sale of its ADS and ordinary shares. The Company has incurred recurring losses since its inception, including net losses of $29.7 million, $12.2 million and $13.6 million for the years ended December 31, 2017, 2016 and 2015, respectively. In addition, as of December 31, 2017 and 2016, the Company had an accumulated deficit of $62.0 million and $32.3 million, respectively. The Company expects to continue to generate operating losses for the foreseeable future. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. The Company’s inability to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. There can be no assurances, however, that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all. The Company believes the cash on hand at December 31, 2017 of $129.4 million will be sufficient to fund the Company’s operations for at least 12 months from the issuance date of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements include those of the Company, NightstaRx Limited, and its U.S. subsidiary, Nightstar, Inc., and have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). All intercompany accounts and transactions have been eliminated upon consolidation. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. 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 consolidated financial statements and the reported amounts of income and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the fair value of ordinary shares, the valuation of tranche obligations, share-based compensation and income taxes. 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 materially from those estimates. Cash and Cash Equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of December 31, 2017 and 2016 consisted primarily of money market funds. Fair Value Measurements Certain assets of the Company are carried at fair value under U.S. 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 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 tranche obligation liability (see Note 7) was carried at fair value, determined according to the fair value hierarchy described above (see Note 3). Management believes that the carrying amounts of the Company’s consolidated financial instruments, including cash equivalents, prepaid expenses, accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments. Concentration of Credit Risk Financial instruments that subject the Company to credit risk consist primarily of cash and cash equivalents. The Company places cash and cash equivalents in established financial institutions. The Company has no significant off-balance-sheet risk or concentration of credit risk, such as foreign exchange contracts, options contracts, or other foreign hedging arrangements. Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2017 and 2016, the Company’s property and equipment consisted of lab equipment, computer equipment and office equipment, which has an estimated useful life of three years. 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 included in the consolidated statement of operations and other comprehensive loss. Expenditures for repairs and maintenance are charged to expense as incurred. The Company evaluates assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2017. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies; however, the Company operates in two geographic regions: United Kingdom and United States. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, depreciation expense, travel, third-party license fees, and external costs of outside vendors engaged to conduct clinical development activities, clinical trials, cost to manufacture clinical trial materials and tax credits associated with research and development activities. Research Contract Costs and Accruals The Company has entered into various research and development-related contracts with research institutions and other companies. 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 or clinical trials, including the phase or completion of events, invoices received and contracted costs. 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 The Company expenses patent application and related legal costs as incurred and classified such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss. Share-Based Compensation The Company grants equity awards under its share-based compensation programs, which may include share options, restricted share awards (“RSAs”), restricted share units (“RSUs”), and other share-based awards. To date, the share-based awards granted to employees and directors have been in the form of RSAs, RSUs and share options. The Company recognizes compensation expense for equity awards based on the grant date fair value of the award. For equity awards that vest based on a service condition, the share-based compensation expense is recognized on a straight-line basis over the requisite service period. The Company uses the fair value of its ordinary shares to determine the fair value of RSAs and RSUs. The fair value of options is determined using the Black-Scholes option pricing model. The Company accounts for forfeitures as they occur. Prior to the IPO, the only share-based compensation issued were in the form of RSAs. The grant date fair value of RSAs were calculated based on the grant date fair value of the underlying ordinary shares. The Company calculated the fair value of the ordinary shares in accordance with the guidelines in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation The OPM backsolve method derives an equity value such that the value indicated for ordinary shares is consistent with the investment price, and it provides an allocation of this equity value to each of the Company’s securities. The OPM treats the various classes of ordinary shares as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, each class of ordinary shares has value only if the funds available for distribution to shareholders exceeded the value of the share liquidation preferences of the class or classes of ordinary shares with senior preferences at the time of the liquidity event. Key inputs into the OPM backsolve calculation included the valuation of forward contracts, expected time to liquidity and volatility. A reasonable discount for lack of marketability was applied to the total equity value to arrive at an estimate of the total fair value of equity on a non-marketable basis. The PWERM is a scenario-based methodology that estimates the fair value of ordinary shares based upon an analysis of future values for the Company, assuming various outcomes. The ordinary share values are based on the probability-weighted present value of expected future investment returns considering each of the possible outcomes available as well as the rights of each class of ordinary shares. The future value of the ordinary shares under each outcome was discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to arrive at an indication of value for the ordinary shares. The hybrid method is a PWERM where the equity value in one of the scenarios is calculated using an OPM. Tranche Obligations The fair values of the tranche obligations are based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The tranche obligations are valued as a forward contract, and the values are determined using a probability-weighted present value calculation. In determining the fair values of the tranche obligations, the inputs impacting fair value included the fair value of the Company’s ordinary shares, risk-free interest rates and the probability and estimated timing of the tranche closings. The Company determines the per share fair value of the underlying ordinary shares using the OPM, which considers the ordinary share price paid by investors, the time to liquidity and volatility. In the OPM, the timing of the liquidity event determines the assumed life in the Black-Scholes calculation. The Company estimates a time to liquidity taking into account the future tranche funding. If the future tranche is not expected to be funded, a liquidity event will be assumed to have occurred. If the tranche is expected to be funded, a longer-term liquidity event is assumed to have occurred. Volatility is estimated based on the daily trading histories of comparable public companies. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve. Foreign Currency Translation The Company maintains its financial statements in the functional currency pounds sterling. The financial statements of the Company’s subsidiary, Nightstar, Inc., are translated from local currency into pounds sterling. Monetary assets and liabilities denominated in currencies other than the functional currency are translated into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are translated into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net income (loss) for the respective periods. The Company recorded foreign exchange losses of $2.9 million, $36,000 and $58,000 for the years ended December 31, 2017, 2016 and 2015, respectively. Foreign exchange gains and losses are included in other expense, net in the consolidated statement of operations and comprehensive loss. For financial reporting purposes, the financial statements of the Company, which are prepared using the functional currency of pounds sterling, have been translated into the U.S. dollar. Assets and liabilities are translated at the exchange rates at the balance sheet dates, revenue and expenses are translated at the average exchange rates and shareholders’ equity is translated based on historical exchange rates. Translation adjustments are not included in determining net income (loss) but are included in foreign exchange adjustment to other comprehensive income (loss), a component of shareholders’ equity. 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 consolidated financial statements or in its tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the consolidated 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 deferred tax assets will be recovered in the future to the extent management 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 by recognizing in the consolidated 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 as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used 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. The Company recognizes interest and penalties related to unrecognized tax benefits on the income tax expense line in the accompanying consolidated statement of operations. As of December 31, 2017 and 2016, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet. Research and development tax credits received from HM Revenue & Customs are recognized as offsets to research and development expenses. Comprehensive Loss The Company follows the provisions of the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 220, Comprehensive Income Net Loss per Share Basic and diluted net loss per ordinary share is determined by dividing net loss by the weighted average number of ordinary shares outstanding during the period. For all periods presented, the outstanding shares of unvested RSAs have been excluded from the calculation because its effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect at December 31, 2017, 2016 and 2015: December 31, 2017 2016 2015 Unvested restricted share awards 1,191,344 894,770 509,533 Emerging Growth Company Status The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act (“JOBS Act”) and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its initial public offering or such earlier time that it is no longer an emerging growth company. The Company would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, has more than $700 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months, and has filed one annual report on Form 20-F), or it issues more than $1 billion of non-convertible debt securities over a three-year period. Ordinary Share Conversion On the date of the IPO, the Company converted its outstanding ordinary shares as discussed in Note 7. All share and per share information has been retroactively adjusted to reflect the share conversion other than items listed under Previous Amendments in Note 7. JOBS Act On April 5, 2012, the Jumpstart Our Business Startups Act, or the JOBS Act, was enacted. The JOBS Act provides that, among other things, an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. As an emerging growth company, the Company has irrevocably elected not to take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards and, as a result, the Company will comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non-emerging growth public companies. In addition, the Company intends to rely on the other exemptions and reduced reporting requirements provided by the JOBS Act. Subject to certain conditions set forth in the JOBS Act, the Company is entitled to rely on certain exemptions as an “emerging growth company.” As an emerging growth company, the Company is not required to, among other things, (i) provide an auditor’s attestation report on the Company’s system of internal controls over financial reporting pursuant to Section 404(b), (ii) provide all of the compensation disclosure that may be required of non-emerging growth public companies under the Dodd-Frank Wall Street Reform and Consumer Protection Act, (iii) comply with any requirement that may be adopted by the Public Company Accounting Oversight Board regarding mandatory audit firm rotation or a supplement to the auditor’s report providing additional information about the audit and the financial statements (auditor discussion and analysis), and (iv) disclose certain executive compensation-related items such as the correlation between executive compensation and performance and comparisons of the chief executive officer’s compensation to median employee compensation. These exemptions will apply for a period of five years following the completion of the IPO or until the Company no longer meets the requirements of being an emerging growth company, whichever is earlier. Recent Accounting Pronouncements Recently issued accounting pronouncements not yet adopted In May 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of Effective Date In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other than Inventory In November 2016, the FASB issued Accounting Standards Update No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash The ASU is effective for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. Early adoption is permitted. In January 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business In January 2017, the FASB issued Accounting Standards Update No. 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment The guidance is effective for annual periods beginning after December 15, 2019 and is applicable to the Company in fiscal 2020. Early adoption is permitted. Recently adopted accounting pronouncements In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (“Topic 718”): Improvements to Employee Share-Based Payment Accounting In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The Company’s previously outstanding tranche obligation liability (see Note 7) is measured at fair value on a recurring basis and is classified as Level 3 in the fair value hierarchy. The following table provides a summary of the changes in fair value of the tranche obligation liability measured at fair value on a recurring basis using significant unobservable inputs during the year ended December 31, 2015 (in thousands): Tranche Obligations Fair value at December 31, 2014 $ 3,883 Change in fair value of tranche obligations 4,333 Reclassification of tranche obligation liability to additional paid-in capital (see Note 7) (8,216 ) Balance at December 31, 2015 $ — The fair values of the tranche obligations (see Note 7) were based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The tranche obligations were valued as a forward contract, and the values were determined using a probability-weighted present value calculation. In determining the fair values of the tranche obligations, the inputs impacting fair value included the fair value of the Company’s Class A ordinary shares, risk-free interest rates and the probability and estimated timing of the tranche closings. The Company determined the per share fair value of the underlying ordinary shares using an option-pricing model (“OPM”), which considers the Class A ordinary share price paid by investors, the time to liquidity and volatility. In the OPM, the timing of the liquidity event determines the assumed life in the Black-Scholes calculation. The Company estimated a time to liquidity taking into account the future tranche funding. If the future tranche was not funded, a liquidity event was assumed to have occurred. If the tranche was funded, a longer-term liquidity event was assumed to have occurred. Volatility was estimated based on the daily trading histories of comparable public companies. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve. Significant changes to the probability or expected timing of the tranche closings, or the fair value of the Company’s Class A ordinary shares, would have resulted in a significant change in the fair value measurement. In November 2015, the tranche obligations were amended and met the requirements to be recorded in equity (see Note 7). Therefore, the Company remeasured the tranche obligation liability to fair value in November 2015, which resulted in an increase in fair value of $4.3 million that was recorded in other expense in the consolidated statement of operations and comprehensive loss. The final valuation of the tranche obligation liability was calculated by separately valuing the portion that was settled and the obligation that remained outstanding. For the portion settled in November 2015, the valuation was calculated as the difference between the price per share paid by investors to settle the portion of the tranche obligation liability (£1.00) and the fair value of the Class A ordinary shares on that date (£1.44), multiplied by the 2.5 million shares issued. For the remaining portion of the tranche obligation liability, the fair value was determined prior to reclassification to equity using an estimated tranche closing date of September 30, 2016, a risk-free rate of 0.42% and an 80% probability of tranche closing. Following the modification, the tranche obligations were considered indexed to the Company’s ordinary shares and met all additional criteria to be classified as equity. Subsequent to the remeasurement in November 2015, the tranche obligation liability was reclassified to additional paid-in capital.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	4. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2017 2016 Prepayments $ 2,586 $ 877 Research and development tax credit 1,753 2,811 Value-added tax receivable 905 327 Interest receivable 98 — Other 96 95 Total prepaid expenses and other current assets $ 5,438 $ 4,110

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2017 2016 Lab equipment $ 363 $ 212 Computer and office equipment 382 311 Less: accumulated depreciation (390 ) (160 ) Total property and equipment, net $ 355 $ 363 Depreciation expense was $206,000, $138,000 and $40,000 for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses and Other Liabilities	6. Accrued Expenses and Other Liabilities Accrued expenses and other liabilities consisted of the following (in thousands): December 31, 2017 2016 Research and development activities $ 3,779 $ 3,692 Compensation and benefits 1,093 376 Professional fees 410 129 Other 907 125 Total accrued expenses and other liabilities $ 6,189 $ 4,322

Shareholders_ Equity

Shareholders’ Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Shareholders’ Equity	7. Shareholders’ Equity Ordinary Shares Each holder of ordinary shares is entitled to one vote per ordinary share and to receive dividends when and if such dividends are recommended by the board of directors and declared by the shareholders. As of December 31, 2017, the Company has not declared any dividends. As of December 31, 2017, the Company’s authorized capital stock consisted of 100 million ordinary shares with a nominal value of £0.01 per share. Initial Public Offering On October 2, 2017, the Company closed its IPO of ADSs. In the IPO, the Company sold an aggregate of 6,164,000 at a public offering price of $14.00 per ADS, including the full exercise by the underwriters of their option to purchase additional ADSs. Net proceeds were approximately $77.4 million, after deducting underwriting discounts, and commissions and offering expenses paid by the Company of $2.9 million. Upon the closing of the IPO, each separate class of ordinary shares of Nightstar Therapeutics plc was converted into a single class of ordinary shares of Nightstar Therapeutics plc as described further below. Corporate Reorganization On September 11, 2017, all shareholders of NightstaRx Limited exchanged each of the ordinary shares of different classes held by them for the same number and class of newly issued ordinary shares of Nightstar Therapeutics Limited and, as a result, NightstaRx became a wholly-owned subsidiary of Nightstar Therapeutics Limited on that date. On September 15, 2017, Nightstar Therapeutics re-registered as a public limited company and was renamed Nightstar Therapeutics plc. The following description summarizes each of the steps of the corporate reorganization taken in connection with the IPO. Exchange of NightstaRx Limited shares for Nightstar Therapeutics Limited shares Prior to the initial step of its corporate reorganization on September 11, 2017, the share capital of NightstaRx Limited consisted of 41,604,393 Class A ordinary shares; 138,596 Class B ordinary shares; 699,282 Class C ordinary shares; 699,802 Class D ordinary shares; 969,520 Class E1 ordinary shares; 481,500 Class E2 ordinary shares; 464,500 Class E3 ordinary shares; 375,500 Class E4 ordinary shares; and 1,854,780 Class F ordinary shares. Re-registration of Nightstar Therapeutics Limited as Nightstar Therapeutics plc Following NightstaRx Limited becoming a wholly owned subsidiary of Nightstar Therapeutics Limited, on September 15, 2017, Nightstar Therapeutics Limited re-registered as a public limited company. Such re-registration required the passing of special resolutions by the shareholders of Nightstar Therapeutics Limited to approve the re-registration as a public limited company, the name change to Nightstar Therapeutics plc and the adoption of new articles of association for Nightstar Therapeutics plc. Conversion of separate classes of ordinary shares of Nightstar Therapeutics plc into single class of ordinary shares Pursuant to the terms of the articles of association of Nightstar Therapeutics plc in effect following its re-registration as a public limited company, on the date of the IPO, each separate class of ordinary shares of Nightstar Therapeutics plc was converted into a single class of ordinary shares of Nightstar Therapeutics plc. The ratio for the conversion of each class of ordinary shares of Nightstar Therapeutics plc into ordinary shares of Nightstar Therapeutics plc was determined based on the IPO price. Based on the IPO price of $14.00 per ADS, each class of the ordinary shares of Nightstar Therapeutics plc outstanding on the date of the closing of the IPO was converted into ordinary shares of Nightstar Therapeutics plc according to the following approximate ratios: ▪ each Class A ordinary share was converted into approximately 0.5051 ordinary shares; ▪ each Class B ordinary share was converted into approximately 0.4122 ordinary shares; ▪ each Class C ordinary share was converted into approximately 0.4122 ordinary shares; ▪ each Class D ordinary share was converted into approximately 0.3751 ordinary shares; ▪ each Class E1, E2, E3 and E4 ordinary share was converted into approximately 0.3125 ordinary shares; and ▪ each Class F ordinary share was converted into approximately 0.2182 ordinary shares. The number of ordinary shares that each shareholder of Nightstar Therapeutics plc received was rounded up or down to the nearest whole share. Therefore, based on the IPO price of $14.00 per ADS, the shareholders of Nightstar Therapeutics plc held an aggregate of 22,740,714 ordinary shares of Nightstar Therapeutics plc at the time of conversion. The Company had the following number of ordinary shares issued and outstanding at December 31, 2017, 2016 and 2015. The table below reflects the conversion of ordinary shares in the current and previous years. December 31, 2017 2016 2015 Class A ordinary shares — 11,277,900 7,941,989 Class B ordinary shares — 57,127 57,127 Class C ordinary shares — 368,821 368,821 Class D ordinary shares — 199,560 199,560 Class E1 ordinary shares — 209,064 — Class E2 ordinary shares — 123,279 — Class E3 ordinary shares — 90,155 — Class E4 ordinary shares — 90,155 — Deferred shares — 129,253 126,128 Ordinary shares 28,904,714 — — Total ordinary shares 28,904,714 12,545,314 8,693,625 Previous amendments As of December 31, 2016 and 2015, the amended and restated Articles of Association, authorized the Company to issue 50,000,000 Class A ordinary shares, 5,000,000 Class B ordinary shares, 5,000,000 Class C ordinary shares, 5,000,000 Class D ordinary shares, 5,000,000 Class E ordinary shares and 5,000,000 Class F ordinary shares. The ordinary shares were originally authorized to be issued by NightstaRx Limited with a nominal value of £0.00001. Subsequent to the corporate reorganization, the Company’s ordinary shares have a nominal value of £0.01. Each ordinary share entitles the holder to vote on all matters submitted to a vote of the Company’s shareholders. Ordinary shareholders are entitled to receive dividends, as may be declared by the Company’s board of directors. A holder may elect to receive non-cash (shares or other assets) dividends. Under the amended and restated Articles of Association in effect at such time, in the event of a distribution of assets on liquidation or a return of capital, the surplus remaining after payment of the Company’s liabilities was to be applied first in paying to each holder of Class A ordinary shares an amount per share held equal to £1.00; second in paying to each holder of Class A, B and C ordinary shares an amount per share held equal to £0.40; third in paying to each holder of Class A, B, C and D ordinary shares an amount per share equal to £0.67366; fourth in paying to each holder of Class A, B, C, D and E ordinary shares an amount per share to the difference between $4.01648 per share and £2.07366 (calculated based on the exchange rate in effect as of the date of determination); and fifth in paying to each holder of Class A, B, C, D, E and F ordinary shares and remaining amounts on a pro rata basis. Any deferred shareholders would have been paid at an aggregate price of £1.00 for all deferred shares held. 2013 Financing and 2014 Amendment In November 2013, in connection with the execution of a subscription agreement (the “2013 Agreement”), the Company issued 1,000,000 Class A ordinary shares for total proceeds of $1.6 million and incurred issuance costs of $22,000. The 2013 Agreement authorized the future issuance of several tranches of Class A ordinary shares upon the Company meeting prespecified milestones. The 2013 Agreement was amended and restated in May 2014 (as so amended, the “Amended 2013 Agreement”). The Amended 2013 Agreement included a modification to the previous milestones. The Company committed to issuing 6,000,000 Class A ordinary shares at a purchase price of £1.00 per share after the completion of both (i) a clinical plan that the Company’s Clinical Advisory Board and Syncona Partners LLP, Syncona Limited and their affiliates (collectively, “Syncona”) agreed would represent a registration package in Europe and the United States and (ii) a manufacturing plan that the Clinical Advisory Board and Syncona agreed would meet European Medicines Agency (“EMA”) and U.S. Food and Drug Administration (“FDA”) standards for a registration product and would be sufficiently scalable to adequately supply the worldwide patient population (“Tranche 2A”). These Tranche 2A milestones were achieved, and the Company issued 6,000,000 Class A ordinary shares in May 2014 for gross proceeds of $10.1 million. Additionally, the Company committed to the issuance of 1,000,000 Class A ordinary shares at a purchase price of £1.00 per share upon presentation by the Company of a plan for a ten-patient study that Syncona determined would enhance the Company’s competitive position or strengthen the clinical plan (“Tranche 2B”). This Tranche 2B milestone was achieved, and the Company issued 1,000,000 Class A ordinary shares in June 2015 for gross proceeds of $1.6 million. Lastly, the Company committed to issuing additional shares upon the completion of both (i) one or more patient studies that enrolled more than 15 patients, or such lesser amount may be agreed, with a one-year efficacy read-out that the investor determined justified the initiation of a pivotal trial and (ii) release of NSR-REP1 that the investor determined met the standard of commercial Good Manufacturing Practice (“cGMP”) as determined by the FDA and EMA (“Tranche 3A”). Further, shares could be issued in a final tranche upon the completion of a material event in the business plan for the Company for the period from completion of the Tranche 3A milestones until 12 months after the Company receives approval for NSR-REP1 in the United States (“Tranche 3B”). The Class A ordinary shares to be issued under Tranche 3A and Tranche 3B were based on a formula which capped the issuance at 9,000,000 shares. The purchase price per share was initially £1.00 per share, but could be adjusted to £1.40 if the Company achieved approval for NSR-REP1 in either the United States or the European Union prior to completion of Tranche 3A, or if the Company’s net cash position was greater than the budgeted operating expense for the following six-month period. The Tranche 3A and 3B milestones were not achieved at the time of the November 2015 financing. The Company’s obligation to issue shares under the Amended 2013 Agreement was modified by the November 2015 financing discussed below. 2015 Financing In November 2015, the Company entered into a subscription and shareholders agreement (the “2015 Agreement”), which provided for the issuance of several tranches of Class A ordinary shares. Tranche 1 did not have any prespecified milestones. Shares for Tranche 1 were issued concurrently with the execution of the 2015 Agreement and resulted in the issuance of 5,225,139 Class A ordinary shares for total proceeds of $12.5 million, and the Company incurred issuance costs of $139,000. The 2015 Agreement modified the prespecified milestone criteria for Tranche 3A and Tranche 3B of the Amended 2013 Agreement such that investors who had the right to purchase shares under Tranche 3A and Tranche 3B were allowed to purchase 2,500,000 shares issued in Tranche 1 and 6,500,000 shares to be issued in Tranche 2 discussed below. Subsequent to the modification of Tranche 3A and Tranche 3B milestones in November 2015, the number of Class A ordinary shares and the purchase price for these modified milestones became fixed. The Company committed to issuing 9,225,139 shares at the commencement of a pivotal trial approved by the board of directors for CHM (“Tranche 2”), of which the 6,500,000 shares included from Tranche 3A and Tranche 3B were offered at a price of £1.00 per share and the remainder for £2.07366 per share. Additionally, the Company committed to issuing 2,725,139 Class A ordinary shares after the first patient was treated in a clinical trial, approved by the board of directors, for a second program (“Tranche 3”) for £2.07366 per share. Lastly, the Company committed to issuing 2,725,139 ordinary shares after the first patient was treated in a clinical trial, approved by the board of directors for a third program (“Tranche 4”) for £2.07366 per share. The Tranche 2, 3 and 4 milestones were not achieved as of December 31, 2015 or 2016. As further discussed below under “Tranche Obligation Liability,” as of December 31, 2018, there were no Tranche Obligations outstanding. Under the 2015 Agreement, the investors could purchase shares prior to the satisfaction of Tranche 2 through Tranche 4 milestones. The investors exercised this right in September 2016, and the Company issued 6,605,103 shares of Class A ordinary shares for gross proceeds of $11.3 million. This issuance was in partial satisfaction of the shares to be issued upon the satisfaction of the Tranche 2 milestone, but the exercise of this right in September 2016 did not modify the preset milestone criteria for Tranche 2. March and June 2017 Financings In March 2017, the Company entered into a subscription and shareholders’ agreement, (the “March 2017 Agreement”), whereby certain shareholders elected to subscribe for an aggregate of 8,070,314 Class A ordinary shares as follows: 1,846,071 Class A ordinary shares at a price of £1.00 per share and 6,224,243 Class A ordinary shares at a price of £2.0737 per share. The Company issued all 8,070,314 Class A ordinary shares pursuant to the March 2017 Agreement on June 27, 2017 for gross proceeds of $18.8 million, and the Company incurred issuance costs of $50,000. The shares were issued to satisfy all of the outstanding tranche obligations, discussed below in this note under “Tranche Obligation Liability.” The March 2017 Agreement was terminated in June 2017. In June 2017, the Company entered into a subscription and shareholders’ agreement, (the “June 2017 Agreement”), pursuant to which the Company issued an aggregate of 11,203,837 Class A ordinary shares at a price of $4.01648 per share for gross proceeds of $45.0 million, and the Company incurred issuance costs of $295,000. The June 2017 Agreement was terminated at the time of the Company’s corporate reorganization and IPO. Tranche Obligation Liability The Company assessed the obligations to issue additional shares as part of the Amended 2013 Agreement and as part of the 2015 Agreement (collectively, the “Tranche Obligations”). The Company concluded that such obligations were freestanding financial instruments that were required to be separately recorded at fair value on the date the financing agreements were executed. With the exception of the shares that were to be issued upon satisfaction of the Tranche 3A and 3B milestones, contemplated by the Amended 2013 Agreement, all other Tranche Obligations were considered indexed to the Company’s ordinary shares, met all additional criteria in order to be classified as equity and were recorded to additional paid-in capital at an amount based on the relative fair value of the Tranche Obligations and Class A ordinary shares upon issuance. The obligations to issue shares upon satisfaction of the Tranche 3A and 3B milestones were not considered indexed to the Company’s ordinary shares due to variability of the settlement amount and was required to be recorded as a liability. This liability was estimated to have a fair value of $4.2 million upon issuance in May 2014 and was subsequently remeasured to its fair value of $3.9 million on December 31, 2014, with the decrease in fair value during the year recorded in other expense in the consolidated statement of operations and comprehensive loss. As discussed above, the 2015 Agreement modified the Tranche 3A and 3B milestones. Therefore, the Company remeasured the Tranche 3A and 3B milestone liabilities to fair value in November 2015, which resulted in an increase in fair value of $4.3 million that was recorded in other expense in the consolidated statement of operations and comprehensive loss. All outstanding Tranche Obligations following the modification of the 2015 Agreement were considered indexed to the Company’s ordinary shares and met all additional criteria to be classified as equity and, subsequent to the remeasurement in November 2015, the Tranche Obligations liability was reclassified to additional paid-in capital. At December 31, 2016, the remaining outstanding Tranche Obligations and their respective prespecified milestone criteria were as follows: Tranche Number of Shares Price Per Share Milestone 2 1,846,070 £ 1.00 Commencement of a pivotal trial for CHM 2 773,966 £ 2.07366 Commencement of a pivotal trial for CHM 3 2,725,139 £ 2.07366 First patient treated in a clinical trial for a second program 4 2,725,139 £ 2.07366 First patient treated in a clinical trial for a third program The Company issued shares in June 2017 to satisfy the outstanding Tranche Obligations and accordingly, there were no Tranche Obligations outstanding as of December 31, 2017. In addition to the issuance of Class A ordinary shares to founders and investors, the Company has issued Class B, C, D, E and F ordinary shares to certain employees as employee incentive awards (see Note 8), all of which were converted into a single class of ordinary shares as part of the corporate reorganization described above.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-Based Compensation	8. Share-Based Compensation 2017 Equity Incentive Plan Under the Company’s 2017 Equity Incentive Plan (“2017 Plan”) approved in September 2017, the Company was authorized to issue a total of Share Option Valuation The assumptions used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees and directors during the year ended December 31, 2017 were as follows: December 31, 2017 Expected option life (years) 6 years Risk-free interest rate 2.16% to 2.25% Expected volatility 57.17% to 57.43% Expected dividend yield 0.00% Share option activity under the 2017 Equity Incentive Plan is as follows: Number of Options Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Life Aggregate Intrinsic Value Outstanding at December 31, 2016 — $ — — $ — Granted 191,275 21.00 — 0.64 Exercised — — — 0.00 Forfeited — — — — Outstanding at December 31, 2017 191,275 $ 21.00 9.87 years $ 21.00 Exercisable as of December 31, 2017 — $ — — $ — Vested and expected to vest as of December 31, 2017 191,275 $ 21.00 9.87 years $ — There were no share options granted during December 31, 2016 and 2015. Employee Incentive Shares The 2013 Agreement and the 2015 Agreement each provided for the grant of employee incentive awards in the form of restricted ordinary shares. At December 31, 2016, the Company was authorized to issue Class B, C, D and E ordinary shares. The Company executed a reverse share split in September 2017, whereby all Class B, C, D and E ordinary shares were converted into a single class of ordinary shares. In September 2017, the Company also executed the “2017 Plan” in accordance with its terms. The Company typically grants employee incentive shares which vest over a four-year service period with 25% of the award vesting on the first anniversary of the vesting commencement date, with the balance vesting periodically over the remaining three years, unless the awards contain specific performance vesting provisions. For employee incentive shares issued that have both a performance vesting condition and a services condition (“Performance Awards”), once the performance criteria is achieved, the awards are then subject to a four-year service vesting with 25% of the award vesting on the first anniversary of the performance condition being achieved, with the balance vesting periodically over the remaining three years. The Company recognizes share-based compensation expense for equity awards based on the grant date fair value of the award. For equity awards that vest based on a service condition, the share-based compensation expense is recognized on a straight-line basis over the requisite service period. For Performance Awards, the Company recognizes share-based compensation expense ratably over the requisite service period when the achievement of a performance-based milestone is probable based on the relative satisfaction of the performance condition as of the reporting date. The unvested RSAs subject to repurchase and unvested RSUs are not considered outstanding shares until the holders perform the requisite services or the holder is no longer an employee of the Company. Unvested RSAs are considered deferred shares and all deferred shares can be repurchased by the Company for payment of £0.01 to the holder of the deferred shares. As of December 31, 2017, the Company had repurchased all deferred shares. The aggregate intrinsic value of share options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s ordinary shares for those share options that had exercise prices lower than the fair value of the Company’s ordinary shares. In all cases the exercise price was greater than the fair value of the Company’s ordinary shares at year end; therefore, there is no intrinsic value on the shares options granted. The weighted average grant-date fair value of share options granted during the year ended December 31, 2017 was $11.56 per share, none of which were vested. There were no share options granted during the year ended December 31, 2016 and 2015. Restricted Ordinary Shares A summary of the changes in the Company’s restricted ordinary shares during the year ended December 31, 2015, 2016 and 2017 are as follows. The table below reflects the conversion of shares in the current and previous years (see Note 7): Number of Restricted Shares Weighted Average Grant Date Fair Value Outstanding at January 1, 2015 481,587 $ 1.01 Granted 144,549 0.64 Vested (116,603 ) 1.24 Forfeited — — Outstanding at December 31, 2015 509,533 0.85 Granted 515,778 2.86 Vested (127,416 ) 1.69 Forfeited (3,125 ) 2.89 Outstanding at December 31, 2016 894,770 1.90 Granted 665,910 7.76 Vested (351,656 ) 1.34 Forfeited (17,680 ) 0.64 Unvested and outstanding at December 31, 2017 1,191,344 $ 4.92 During the year ended December 31, 2015, the Company granted an aggregate of 145,000 restricted employee incentive shares to its employees and directors, of which (i) 12,000 restricted employee incentive shares were Class C ordinary shares and contained only time-based service conditions and (ii) 133,000 restricted employee incentive shares were Class D ordinary shares and contained both performance and service based conditions. These 2015 performance-based restricted employee incentive shares begin to vest upon the Company achieving specific research and development milestones, which were linked to the Company commencing a pivotal trial for CHM (see Note 7). The Company determined that achievement of the Tranche 2 milestones related to the 2015 performance-based restricted employee incentive shares was probable as of December 31, 2017, 2016 and 2015 and accordingly, recorded share compensation expense of $8,000, $13,000 and $11,000 for the years ended December 31, 2017, 2016 and 2015, respectively. During the year ended December 31, 2016, the Company granted 516,000 restricted employee incentive shares to its employees and directors. The restricted employee incentive shares were issued in four different classes of the Company’s Class E ordinary shares. For the restricted shares granted in 2016, (i) 212,000 restricted shares were the Company’s Class E1 ordinary shares and contained only time-based service conditions shares and (ii) 124,000, 90,000 and 90,000 shares were the Company’s Class E2, Class E3 and Class E4 ordinary shares, respectively, and contained both performance and service based conditions. The Company determined that achievement of all milestones related to the 2016 performance-based restricted employee incentive shares were probable as of December 31, 2017, and 2016 and recorded share-based compensation expense of $242,000 and $106,000 during the year ended December 31, 2017 and 2016, respectively. During the year ended December 31, 2017, the Company granted 666,000 restricted employee incentive shares to its employees and directors. The restricted employee incentive shares were issued in four different classes of the Company’s Class E ordinary shares. For the restricted shares granted in 2017, (i) 94,000 restricted shares were the Company’s Class E1 ordinary shares and contained only service-based vesting conditions and (ii) 27,000, 55,000 and 27,000 shares were the Company’s Class E2, Class E3 and Class E4 ordinary shares, respectively, which contained both performance- and service-based conditions. Additionally, the Company granted Class F ordinary shares of 405,000 and granted RSUs of 58,000 that contained service-based vesting conditions only. The Company determined that the achievement of the performance-based conditions related to the 2017 performance-based restricted employee incentive shares was probable as of December 31, 2017. These 2016 and 2017 performance-based restricted employee incentive shares begin to vest upon the Company achieving specific research and development milestones. The milestones of the Class E2 ordinary shares were linked to the Company commencing a pivotal trial for CHM (see Note 7). The milestones of the Class E3 ordinary shares were linked to treatment of the first patient in a clinical trial for a second program (see Note 7). The milestones of the Class E4 ordinary shares were linked to treatment of the first patient in a clinical trial for a third program (see Note 7). The Company recorded share-based compensation expense of $58,000 at December 31, 2017 for the 2017 performance and service based restricted shares. In June 2017, in conjunction with the June 2017 Agreement, the Company accelerated the vesting commencement dates of the Class D, Class E2, Class E3 and Class E4 ordinary shares. The vesting commencement dates are the later of the grant date or May 1, 2015, June 6, 2017, April 4, 2017 and June 6, 2017 for the Class D, Class E2, Class E3 and Class E4 ordinary shares, respectively. The Company will amortize the unvested shares and recognize share-based compensation expense over the new accelerated vesting periods. In December 2017, the Company further modified the vesting commencement dates to allow the vesting of awards to begin at the beginning of the quarter in which the vesting would have otherwise occurred, which the impact was $0.5 million for the year ended December 31, 2017. In September 2017, the Company effected a reverse share split (see Note 7), which resulted in the conversion of all Class B, C, D and E ordinary shares into a single class of ordinary shares. The Company recorded share-based compensation expense of $1.4 million, $208,000 and $112,000 during the years ended December 31, 2017, 2016 and 2015, respectively. Share-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands): December 31, 2017 2016 2015 Research and development $ 479 $ 152 $ 81 General and administrative 894 56 31 Total share-based compensation $ 1,373 $ 208 $ 112 In July 2017, the Company issued 36,000 Class F ordinary shares to two employees at the nominal value of £0.00001 per share. In August 2017, the Company issued 369,000 Class F ordinary shares, consisting of (i) 309,000 Class F ordinary shares to five employees at the nominal value of £0.00001 per share, including 133,000 Class F ordinary shares to the Company’s Chief Executive Officer, 130,000 Class F ordinary shares to the Company’s Chief Development Officer, 22,000 Class F ordinary shares to the Company’s Chief Scientific Officer and 21,000 Class F ordinary shares to the Company’s Chief Financial Officer; and (ii) 60,000 Class F ordinary shares to three members of the Company’s board of directors at the nominal value of £0.00001 per share. Subsequent to the corporate reorganization, the Company’s ordinary shares have a nominal value of £0.01.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2017
License Agreements [Abstract]
License Agreements	9. License Agreements Oxford University Innovation Limited Licenses In November 2013, the Company entered an exclusive license agreement (the “2013 Oxford Agreement”) with Oxford University Innovation (“Oxford”), formerly Isis Innovation Limited, to obtain a license to certain patent rights for the commercial development, manufacture, distribution, use and sale of products and processes resulting from the development of those patent rights. The Company is using the licensed patent rights to develop NSR-REP1 for CHM. As part of the consideration for this license, the Company paid upfront fees of $78,000 and past patent cost of $68,000. The 2013 Oxford Agreement requires the Company to remit fees upon the Company or any sub-licensee meeting certain milestones, as well as up to an aggregate of £375,000 ($507,000) upon the achievement of specified milestones and a low single-digit percentage of net of sales upon the Company achieving regulatory approval, subject to quarterly minimums. In addition, the Company agreed to pay a mid single-digit percentage of all upfront fees, milestone and other one-off payments received by the Company. In November 2015, the Company entered into five separate exclusive license agreements (the “2015 Oxford Agreements”) to obtain a license to certain patent rights for the commercial development, manufacture, distribution, use and sale of gene therapy products targeting five different types of inherited retinal diseases, including NSR-RPGR and NSR-BEST1, and processes resulting from the development of those patent rights. As part of the consideration for these licenses, the Company paid upfront fees of $575,000 in the aggregate. The Company also paid past patent costs of $2,000 related to the licenses for RPGR. In addition, the Company agreed to pay Oxford annual maintenance fees for all five licenses until the formal application for regulatory approval of the product is filed, as well as upon the achievement of specified development and commercial milestones and a fee income royalty, which is calculated as a single-digit percentage of all upfront fees, milestone and other one-off payments received by the Company. The Company is also required to pay royalty payments, subject to quarterly minimums, based on a single digit percentage of net sales. In connection with the licenses, Oxford was required to transfer certain manufactured license products as well as all manufacturing documentation. The annual maintenance fee for all five licenses is £50,000 ($68,000) for the year ending December 31, 2017 and will increase to £100,000 ($135,000) through 2025 and beyond. In October 2017, the Company entered into a separate exclusive license agreement (the “2017 Oxford Agreement”) , including NSR-ABCA4, for the treatment of Stargardt disease. As part of the consideration for this license, the Company paid Oxford an upfront signing fee of £100,000 ($135,000). In addition, the Company agreed to pay Oxford annual maintenance fees and payments upon the achievement of specified development and commercial milestones, sales milestones based on the first achievement of predefined sales thresholds, a royalty on annual net sales, and a fee income royalty, which is calculated as a high single-digit percentage of all upfront fees, milestone and other one-off payments received by the Company. In connection with the licenses, Oxford was required to transfer all manufacturing documentation. No annual maintenance fee was due for the year ending December 31, 2017 and will increase to £10,000 in 2018 and up to £20,000 through 2025 and beyond. In the event that the Company commits a material breach, has a petition presented for winding up of the business or passes a resolution for voluntary winding up or challenges the licensed patent rights of Oxford, Oxford can terminate the applicable agreement. The Company has the right to terminate the 2013 Oxford Agreement and 2015 Oxford Agreement if Oxford commits a material breach and at any time after the second anniversary of the applicable agreement. The Company has the right to terminate the 2017 Oxford Agreement if Oxford commits a material breach and at any time after the third anniversary of its execution date upon two months’ written notice. The 2013 Oxford Agreement, 2015 Oxford Agreements and 2017 Oxford Agreement expire on the twentieth anniversary of the applicable effective date or each license will expire upon the Company’s written election when for twelve consecutive months none of these items exist: the licensed patents continue to be in force or subsisting in another country, the Company has market exclusivity throughout the world, the Company has market exclusivity throughout any region or the Company has market exclusivity in any country. The Company can sublicense its rights under the 2013 Oxford Agreement, 2015 Oxford Agreements and 2017 Oxford Agreement. The Company recorded research and development expenses of $0, $0 and $577,000 for the years ended December 31, 2017, 2016 and 2015, respectively, in connection with the 2015 Oxford Agreements. There were no payments made for the years ended December 31, 2017, 2016 and 2015 related to the 2013 Oxford Agreement. Oxford BioMedica License In December 2013, the Company entered into a non-exclusive license agreement (the “BioMedica License Agreement”) with Oxford BioMedica (UK) Limited (“Oxford BioMedica”) to obtain a license to certain patent rights for the commercial development, manufacture, distribution, use and sale of products and processes resulting from the development of those patent rights. As part of the consideration for this license, the Company paid upfront fees of $100,000. In addition, the Company is also required to pay quarterly royalty payments based on net sales at a low-single digit percentage. The BioMedica License Agreement also requires the Company to make a $100,000 milestone payment upon the grant by the FDA of marketing approval for an AAV product for the treatment of CHM. The Company is allowed to sublicense its rights under the agreement, if this occurs the Company is required to pay an upfront sublicense fee and royalty payments equal to a low-to-mid single-digit amount quarterly as a percentage of sublicense revenue. In the event that the Company commits a material breach, challenges the validity of the licensed patent rights, becomes insolvent or if an order is passed or resolution is passed for the winding up of the Company or if an administrator, administrative receiver or receiver is appointed over the Company’s assets, Oxford BioMedica can terminate the BioMedica License Agreement. The Company can terminate the BioMedica License Agreement at any time and if Oxford BioMedica commits a material breach or becomes insolvent or if an order is passed or resolution is passed for the winding up of Oxford BioMedica or if an administrator, administrative receiver or receiver is appointed over Oxford BioMedica’s assets. The BioMedica License Agreement will expire on the expiration date of the related patent. The Company has not recorded any research and development expense for the years ended December 31, 2017, 2016 and 2015 in connection with the BioMedica License Agreement as the Company have not met any milestones and incurred net sales, which would require the Company to remit payments.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The components of loss before provision for income taxes for the years ended December 31, 2017, 2016 and 2015 are as follows (in thousands): December 31, 2017 2016 2015 U.K. $ (29,978 ) $ (12,194 ) $ (13,648 ) U.S. 329 (4 ) — Loss before taxes $ (29,649 ) $ (12,198 ) $ (13,648 ) The Company did not record any income tax provision for the years ended December 31, 2016 and 2015. For the year ended December 31, 2017, the Company recorded the following income tax provision as follows (in thousands): December 31, 2017 Current expense (benefit): Federal $ 52 State — Foreign — Total current expense (benefit) 52 Deferred expense (benefit): Federal — State (15 ) Foreign — Total deferred tax expense (benefit) (15 ) Total income tax expense (benefit) $ 37 A reconciliation of income tax expense (benefit) at the statutory corporate income tax rate to the income tax expense (benefit) at the Company’s effective income tax rate is as follows (in thousands): December 31, 2017 2016 2015 Tax (benefit) at statutory corporate rate $ (5,707 ) $ (2,439 ) $ (2,764 ) Research and development expense 869 835 800 Valuation allowance 4,304 1,122 806 Tranche obligation liability — — 780 Change in tax law 537 437 356 State taxes, net of federal benefit (15 ) — — Foreign rate differential 49 — — Other — 45 22 Income tax expense (benefit) $ 37 $ — $ — Net deferred tax assets (liabilities) recognized in the Company’s consolidated balance sheet consist of the following (in thousands): December 31, 2017 2016 2015 Deferred tax assets: Net operating loss carryforwards $ 6,549 $ 2,109 $ 1,273 Intangibles amortization 18 19 28 Tax credits 15 — Other 8 7 15 Total deferred tax assets 6,590 2,135 1,316 Deferred tax liabilities: Depreciation and amortization (52 ) (51 ) (45 ) Total deferred tax liabilities (52 ) (51 ) (45 ) Less: valuation allowance (6,523 ) (2,084 ) (1,271 ) Net deferred tax asset (liability) $ 15 $ — $ — As a company that carries out extensive research and development activities, the Company seeks to benefit from one or other of two U.K. research and development tax credit cash rebate regimes, the Small and Medium-sized Enterprises R&D Tax Credit Scheme (“SME Scheme”) and the and the Research and Development Expenditure scheme (“RDEC Scheme”). Under the SME Scheme, the Company’s principal research subsidiary company, NightstaRx Limited, may be eligible to surrender the trading losses that arise from its research and development activities for a payable tax credit of up to approximately 33.4% of eligible research and development expenditures. Qualifying expenditures largely comprise employment costs for research staff, consumables and certain internal overhead costs incurred as part of research projects for which the Company does not receive income. Subcontracted research expenditures are eligible for a cash rebate of up to approximately 21.7%. For certain periods where NightstaRx Limited is not eligible for the SME Scheme, NightstaRx Limited is eligible for the RDEC Scheme, whereby tax relief is given at 11% of allowable research and development costs, increasing to 12% from January 1, 2018. The RDEC plan is more restrictive with qualifying expenditures mainly comprising of employment cost for research staff. Based on criteria established by HMRC, management of NightstaRx Limited expects a proportion of expenditures being carried out by NightstaRx Limited in relation to its pipeline research, clinical trials management and manufacturing development activities is likely to be eligible for inclusion within one of these two tax credit cash rebate regimes. In consideration of correspondence with HMRC on most recent claims submitted, management of NightstaRx Limited have completed a reassessment of which scheme is applicable and the eligible costs recorded. The Company has recorded U.K. research and development tax credit as an offset to research and development expense in the consolidated statements of operations and comprehensive loss of $2.5 million, $2.2 million for the years ended December 31, 2015 and 2016. For the year ended December 31, 2017, the Company recorded an expense of $1.3 million to reduce R&D claims payable in accordance with correspondence with HMRC. The Company has evaluated the positive and negative evidence bearing on 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 net deferred tax assets of December 31, 2017, 2016 and 2015. Management reevaluates the positive and negative evidence at each reporting period. As of December 31, 2017, 2016 and 2015, the Company had $37.9 million, $12.4 million and $7.1 million, respectively, of gross U.K. net operating loss carryforwards, which do not expire. The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2017, 2016 and 2015. The Company files income tax returns in the United Kingdom, United States and certain state and local jurisdictions. The income tax returns are generally subject to tax examinations for the tax years ended December 31, 2013 through December 31, 2017. There are currently no pending income tax return examinations.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies License Agreements The Company has entered into several license agreements (see Note 9). In connection with these agreements the Company is required to make a number of milestone payments and annual license maintenance payments. The Company evaluated all milestone payments within the arrangements to estimate the probability of the Company meeting the milestones in accordance with ASC 450, Contingencies Legal Proceedings From time to time, the Company may be a party to litigation or subject to claims incident to the ordinary course of business. Although the results of litigation and claims cannot be predicted with certainty, the Company currently believes that the final outcome of these ordinary course matters will not have a material adverse effect on its business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors. The Company was not a party to any litigation and did not have contingency reserves established for any liabilities as of December 31, 2017 and 2016. Leases The Company’s corporate headquarters is located on Euston Road, London, United Kingdom, for which, as of December 31, 2017 and 2016, the Company leased space under a cancelable lease that can be terminated by either party with three months’ advanced notice. The lease was executed with The Wellcome Trust, which is the prior holding company of Syncona. Syncona is the principal investor in the Company. The lease related to this facility is classified as an operating lease. The Company entered into a noncancelable sublease on January 10, 2017 for a facility in Lexington, Massachusetts with GeNO LLC for its U.S. operations. The lease related to the facility commenced on February 1, 2017 and is scheduled to terminate in June 2020. The lease is classified as an operating lease. The Company is committed to making aggregate lease payments of $86,000 in 2018, $89,000 in 2019 and $46,000 in 2020. The Company recorded rent expense totaling $205,000, $138,000 and $106,000 for the years ended December 31, 2017, 2016 and 2015, respectively. Indemnification In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnification. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations. In accordance with its Articles of Association, the Company has indemnification obligations to its officers and directors for certain events or occurrences, subject to certain limits, while they are serving at the Company’s request in such capacity. There have been no claims to date, and the Company has director and officer insurance that may enable it to recover a portion of any amounts paid for future potential claims. Credit Arrangement The Company has a line of credit arrangement with a bank that allows the Company to submit payments up to £300,000 per month through the Bankers Automated Clearing Services (BACS) system. The arrangement does not require the Company to pay any interest or fees and will remain effective until terminated by either party. At December 31, 2017 and December 31, 2016, there were no amounts outstanding under the arrangement.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions Syncona The Company receives accounting and professional services from its principal investor, Syncona from time to time as needed. The Company recorded accounting and professional fees totaling $1,200, $23,000 and $52,000 for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017, the Company had $1,300 outstanding to Syncona. The Company had no outstanding payables as of December 31, 2016. University of Oxford and Related Entities The Company, under various service agreements, receives research and development services from the University of Oxford and its subsidiaries. The University of Oxford is a shareholder of the Company. The Company recorded research and development expenses totaling $862.000, $376,000 and $871,000 for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017 and 2016 there was $414,000 and $248,000, respectively, included in accrued expenses. Freeline Therapeutics Freeline Therapeutics is a majority-owned portfolio company of Syncona and provided certain ancillary research and development services to the Company through a consulting contract in 2017 and 2016. The Company recorded research and development expenses totaling $11,000 and $30,000 for the years ended December 31, 2017 and 2016, respectively. The Company did not incur any similar expenses in 2015. As of December 31, 2017 and 2016, the Company had $12,200 and $11,000, respectively, outstanding to Freeline Therapeutics. The Wellcome Trust The Company has a lease agreement with The Wellcome Trust Limited, as trustee of The Wellcome Trust, the prior holding company of Syncona (see Note 11). As part of that arrangement, the Company is billed for certain office costs in addition to the leased space. The Company recorded general and administrative expenses totaling $165,000, $152,000 and $182,000 for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017 and 2016, there was $182,000 and $44,000, respectively, outstanding to The Wellcome Trust. Subsequent to December 31, 2016, The Wellcome Trust is no longer considered an affiliated entity of the Company. Gyroscope Therapeutics In January 2018, the Company entered into an agreement with Gyroscope Therapeutics Limited (“Gyroscope”), an entity controlled by Syncona. Pursuant to the agreement, the Company provided Gyroscope with certain clinical manuals and other documents. MacLaren Agreement In November 2013 and as amended in 2016, the Company entered into a consulting agreement with Oxford University Innovation Ltd. (“Consulting Agreement”), for consulting services of Prof. Robert MacLaren, a member of the Company’s board of directors. Under the terms of the Consulting Agreement, Prof. MacLaren is to provide the Company with advice and expertise in relation to regulatory submissions, prepare for and attend meetings of the Company’s clinical advisory board, prepare for and attend regulatory meetings, provide scientific and medical advice in relation to the preparation of medical education materials and provide consulting services to the Company’s Chief Medical Officer. Under the terms of the Consulting Agreement, the Company agreed to pay consulting fees for Prof. MacLaren’s services subject to a minimum annual fee of £72,000 ($97,000). In October 2017, the board of directors authorized an increase in the minimum annual consultancy fee to £99,000 ($134,000). In October 2017, the Company’s board of directors awarded Prof. MacLaren 58,000 restricted share units which vest annually over a four year period from the date of award. The Company has further agreed to pay an annual bonus in respect of Prof. MacLaren’s services based on the Company’s performance as set forth in the Consulting Agreement. The Consulting Agreement terminates in November 2019, unless an extension is mutually agreed upon. Either party can terminate the Consulting Agreement without cause upon six months’ written notice to the other, or such shorter period as may be agreed between the Company and Oxford University Innovation Ltd. The Company can terminate the Consulting Agreement immediately by written notice and without compensation for cause, including where the services are provided negligently or otherwise not in accordance with the Consulting Agreement, or where either Prof. MacLaren or Oxford University Innovation Ltd. commits any material breach of the Consulting Agreement or commits any gross misconduct affecting the Company’s business. The Company recorded consulting fees totaling $107,000, $150,000 and $90,000 for the years ended December 31, 2017, 2016 and 2015, respectively, under the Consulting Agreement. As of December 31, 2017, there was $8,000 outstanding to Oxford University Innovation Ltd. Management Rights Letter In November 2015, in connection with the 2015 Agreement, the Company entered into a management rights letter with New Enterprise Associate 15, L.P. and NEA Ventures 2015, L.P. (collectively, “NEA”), which holds more than 5% of the Company’s share capital. Pursuant to the 2015 Agreement, NEA is entitled to consult and advise management on significant business issues. The management rights letter terminated in September 2017, upon the completion of the IPO. Initial Public Offering In September 2017, entities affiliated with Syncona Portfolio Limited and New Enterprise Associates purchased 1,000,000 and 714,285 ADSs, respectively, in the IPO.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	13. Employee Benefit Plans In the United Kingdom, the Company makes contributions to private defined benefit pension schemes on behalf of its employees. The Company paid $0.2 million, $0.2 million and $0.1 million in contributions in the years ended December 31, 2017, 2016 and 2015, respectively. In the United States, the Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all U.S. employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. The Company does not currently match employee contributions and accordingly, no matching contributions were recorded for the years ended December 31, 2017, 2016 and 2015.

Summary of Significant Accoun20

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include those of the Company, NightstaRx Limited, and its U.S. subsidiary, Nightstar, Inc., and have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). All intercompany accounts and transactions have been eliminated upon consolidation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. 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 consolidated financial statements and the reported amounts of income and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the fair value of ordinary shares, the valuation of tranche obligations, share-based compensation and income taxes. 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 materially from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of December 31, 2017 and 2016 consisted primarily of money market funds.
Fair Value Measurements	Fair Value Measurements Certain assets of the Company are carried at fair value under U.S. 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 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 tranche obligation liability (see Note 7) was carried at fair value, determined according to the fair value hierarchy described above (see Note 3). Management believes that the carrying amounts of the Company’s consolidated financial instruments, including cash equivalents, prepaid expenses, accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that subject the Company to credit risk consist primarily of cash and cash equivalents. The Company places cash and cash equivalents in established financial institutions. The Company has no significant off-balance-sheet risk or concentration of credit risk, such as foreign exchange contracts, options contracts, or other foreign hedging arrangements.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2017 and 2016, the Company’s property and equipment consisted of lab equipment, computer equipment and office equipment, which has an estimated useful life of three years. 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 included in the consolidated statement of operations and other comprehensive loss. Expenditures for repairs and maintenance are charged to expense as incurred. The Company evaluates assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2017.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies; however, the Company operates in two geographic regions: United Kingdom and United States.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, depreciation expense, travel, third-party license fees, and external costs of outside vendors engaged to conduct clinical development activities, clinical trials, cost to manufacture clinical trial materials and tax credits associated with research and development activities.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development-related contracts with research institutions and other companies. 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 or clinical trials, including the phase or completion of events, invoices received and contracted costs. 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 The Company expenses patent application and related legal costs as incurred and classified such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.
Share-Based Compensation	Share-Based Compensation The Company grants equity awards under its share-based compensation programs, which may include share options, restricted share awards (“RSAs”), restricted share units (“RSUs”), and other share-based awards. To date, the share-based awards granted to employees and directors have been in the form of RSAs, RSUs and share options. The Company recognizes compensation expense for equity awards based on the grant date fair value of the award. For equity awards that vest based on a service condition, the share-based compensation expense is recognized on a straight-line basis over the requisite service period. The Company uses the fair value of its ordinary shares to determine the fair value of RSAs and RSUs. The fair value of options is determined using the Black-Scholes option pricing model. The Company accounts for forfeitures as they occur. Prior to the IPO, the only share-based compensation issued were in the form of RSAs. The grant date fair value of RSAs were calculated based on the grant date fair value of the underlying ordinary shares. The Company calculated the fair value of the ordinary shares in accordance with the guidelines in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation The OPM backsolve method derives an equity value such that the value indicated for ordinary shares is consistent with the investment price, and it provides an allocation of this equity value to each of the Company’s securities. The OPM treats the various classes of ordinary shares as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, each class of ordinary shares has value only if the funds available for distribution to shareholders exceeded the value of the share liquidation preferences of the class or classes of ordinary shares with senior preferences at the time of the liquidity event. Key inputs into the OPM backsolve calculation included the valuation of forward contracts, expected time to liquidity and volatility. A reasonable discount for lack of marketability was applied to the total equity value to arrive at an estimate of the total fair value of equity on a non-marketable basis. The PWERM is a scenario-based methodology that estimates the fair value of ordinary shares based upon an analysis of future values for the Company, assuming various outcomes. The ordinary share values are based on the probability-weighted present value of expected future investment returns considering each of the possible outcomes available as well as the rights of each class of ordinary shares. The future value of the ordinary shares under each outcome was discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to arrive at an indication of value for the ordinary shares. The hybrid method is a PWERM where the equity value in one of the scenarios is calculated using an OPM.
Tranche Obligations	Tranche Obligations The fair values of the tranche obligations are based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The tranche obligations are valued as a forward contract, and the values are determined using a probability-weighted present value calculation. In determining the fair values of the tranche obligations, the inputs impacting fair value included the fair value of the Company’s ordinary shares, risk-free interest rates and the probability and estimated timing of the tranche closings. The Company determines the per share fair value of the underlying ordinary shares using the OPM, which considers the ordinary share price paid by investors, the time to liquidity and volatility. In the OPM, the timing of the liquidity event determines the assumed life in the Black-Scholes calculation. The Company estimates a time to liquidity taking into account the future tranche funding. If the future tranche is not expected to be funded, a liquidity event will be assumed to have occurred. If the tranche is expected to be funded, a longer-term liquidity event is assumed to have occurred. Volatility is estimated based on the daily trading histories of comparable public companies. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve.
Foreign Currency Translation	Foreign Currency Translation The Company maintains its financial statements in the functional currency pounds sterling. The financial statements of the Company’s subsidiary, Nightstar, Inc., are translated from local currency into pounds sterling. Monetary assets and liabilities denominated in currencies other than the functional currency are translated into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are translated into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net income (loss) for the respective periods. The Company recorded foreign exchange losses of $2.9 million, $36,000 and $58,000 for the years ended December 31, 2017, 2016 and 2015, respectively. Foreign exchange gains and losses are included in other expense, net in the consolidated statement of operations and comprehensive loss. For financial reporting purposes, the financial statements of the Company, which are prepared using the functional currency of pounds sterling, have been translated into the U.S. dollar. Assets and liabilities are translated at the exchange rates at the balance sheet dates, revenue and expenses are translated at the average exchange rates and shareholders’ equity is translated based on historical exchange rates. Translation adjustments are not included in determining net income (loss) but are included in foreign exchange adjustment to other comprehensive income (loss), a component of shareholders’ equity.
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 consolidated financial statements or in its tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the consolidated 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 deferred tax assets will be recovered in the future to the extent management 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 by recognizing in the consolidated 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 as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used 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. The Company recognizes interest and penalties related to unrecognized tax benefits on the income tax expense line in the accompanying consolidated statement of operations. As of December 31, 2017 and 2016, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet. Research and development tax credits received from HM Revenue & Customs are recognized as offsets to research and development expenses.
Comprehensive Loss	Comprehensive Loss The Company follows the provisions of the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 220, Comprehensive Income
Net Loss per Share	Net Loss per Share Basic and diluted net loss per ordinary share is determined by dividing net loss by the weighted average number of ordinary shares outstanding during the period. For all periods presented, the outstanding shares of unvested RSAs have been excluded from the calculation because its effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect at December 31, 2017, 2016 and 2015: December 31, 2017 2016 2015 Unvested restricted share awards 1,191,344 894,770 509,533
Emerging Growth Company Status	Emerging Growth Company Status The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act (“JOBS Act”) and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its initial public offering or such earlier time that it is no longer an emerging growth company. The Company would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, has more than $700 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months, and has filed one annual report on Form 20-F), or it issues more than $1 billion of non-convertible debt securities over a three-year period.
Ordinary Share Conversion	Ordinary Share Conversion On the date of the IPO, the Company converted its outstanding ordinary shares as discussed in Note 7. All share and per share information has been retroactively adjusted to reflect the share conversion other than items listed under Previous Amendments in Note 7.
JOBS Act	JOBS Act On April 5, 2012, the Jumpstart Our Business Startups Act, or the JOBS Act, was enacted. The JOBS Act provides that, among other things, an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. As an emerging growth company, the Company has irrevocably elected not to take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards and, as a result, the Company will comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non-emerging growth public companies. In addition, the Company intends to rely on the other exemptions and reduced reporting requirements provided by the JOBS Act. Subject to certain conditions set forth in the JOBS Act, the Company is entitled to rely on certain exemptions as an “emerging growth company.” As an emerging growth company, the Company is not required to, among other things, (i) provide an auditor’s attestation report on the Company’s system of internal controls over financial reporting pursuant to Section 404(b), (ii) provide all of the compensation disclosure that may be required of non-emerging growth public companies under the Dodd-Frank Wall Street Reform and Consumer Protection Act, (iii) comply with any requirement that may be adopted by the Public Company Accounting Oversight Board regarding mandatory audit firm rotation or a supplement to the auditor’s report providing additional information about the audit and the financial statements (auditor discussion and analysis), and (iv) disclose certain executive compensation-related items such as the correlation between executive compensation and performance and comparisons of the chief executive officer’s compensation to median employee compensation. These exemptions will apply for a period of five years following the completion of the IPO or until the Company no longer meets the requirements of being an emerging growth company, whichever is earlier.
Recently Issued Accounting Pronouncements Not yet Adopted	Recently issued accounting pronouncements not yet adopted In May 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of Effective Date In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other than Inventory In November 2016, the FASB issued Accounting Standards Update No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash The ASU is effective for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. Early adoption is permitted. In January 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business In January 2017, the FASB issued Accounting Standards Update No. 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment The guidance is effective for annual periods beginning after December 15, 2019 and is applicable to the Company in fiscal 2020. Early adoption is permitted.
Recently Adopted Accounting Pronouncements	Recently adopted accounting pronouncements In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (“Topic 718”): Improvements to Employee Share-Based Payment Accounting In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of Anti-Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share	The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect at December 31, 2017, 2016 and 2015: December 31, 2017 2016 2015 Unvested restricted share awards 1,191,344 894,770 509,533

Fair Value of Financial Asset22

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Summary of Changes in Fair Value of Tranche Obligation Liability Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs	The following table provides a summary of the changes in fair value of the tranche obligation liability measured at fair value on a recurring basis using significant unobservable inputs during the year ended December 31, 2015 (in thousands): Tranche Obligations Fair value at December 31, 2014 $ 3,883 Change in fair value of tranche obligations 4,333 Reclassification of tranche obligation liability to additional paid-in capital (see Note 7) (8,216 ) Balance at December 31, 2015 $ —

Prepaid Expenses and Other Cu23

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2017 2016 Prepayments $ 2,586 $ 877 Research and development tax credit 1,753 2,811 Value-added tax receivable 905 327 Interest receivable 98 — Other 96 95 Total prepaid expenses and other current assets $ 5,438 $ 4,110

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Components of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2017 2016 Lab equipment $ 363 $ 212 Computer and office equipment 382 311 Less: accumulated depreciation (390 ) (160 ) Total property and equipment, net $ 355 $ 363

Accrued Expenses and Other Li25

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Components of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consisted of the following (in thousands): December 31, 2017 2016 Research and development activities $ 3,779 $ 3,692 Compensation and benefits 1,093 376 Professional fees 410 129 Other 907 125 Total accrued expenses and other liabilities $ 6,189 $ 4,322

Shareholders_ Equity (Tables)

Shareholders’ Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Summary of Number of Ordinary Shares Issued and Outstanding	The Company had the following number of ordinary shares issued and outstanding at December 31, 2017, 2016 and 2015. The table below reflects the conversion of ordinary shares in the current and previous years. December 31, 2017 2016 2015 Class A ordinary shares — 11,277,900 7,941,989 Class B ordinary shares — 57,127 57,127 Class C ordinary shares — 368,821 368,821 Class D ordinary shares — 199,560 199,560 Class E1 ordinary shares — 209,064 — Class E2 ordinary shares — 123,279 — Class E3 ordinary shares — 90,155 — Class E4 ordinary shares — 90,155 — Deferred shares — 129,253 126,128 Ordinary shares 28,904,714 — — Total ordinary shares 28,904,714 12,545,314 8,693,625
Schedule of Remaining Outstanding Tranche Obligations and Their Respective Prespecified Milestone Criteria	At December 31, 2016, the remaining outstanding Tranche Obligations and their respective prespecified milestone criteria were as follows: Tranche Number of Shares Price Per Share Milestone 2 1,846,070 £ 1.00 Commencement of a pivotal trial for CHM 2 773,966 £ 2.07366 Commencement of a pivotal trial for CHM 3 2,725,139 £ 2.07366 First patient treated in a clinical trial for a second program 4 2,725,139 £ 2.07366 First patient treated in a clinical trial for a third program

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Assumptions Used in Black-Scholes Option Pricing Model To Determine Fair Value Of Share Options Granted	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees and directors during the year ended December 31, 2017 were as follows: December 31, 2017 Expected option life (years) 6 years Risk-free interest rate 2.16% to 2.25% Expected volatility 57.17% to 57.43% Expected dividend yield 0.00%
Schedule of Share Options	Share option activity under the 2017 Equity Incentive Plan is as follows: Number of Options Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Life Aggregate Intrinsic Value Outstanding at December 31, 2016 — $ — — $ — Granted 191,275 21.00 — 0.64 Exercised — — — 0.00 Forfeited — — — — Outstanding at December 31, 2017 191,275 $ 21.00 9.87 years $ 21.00 Exercisable as of December 31, 2017 — $ — — $ — Vested and expected to vest as of December 31, 2017 191,275 $ 21.00 9.87 years $ —
Summary of Changes in the Company's Restricted Ordinary Shares	A summary of the changes in the Company’s restricted ordinary shares during the year ended December 31, 2015, 2016 and 2017 are as follows. The table below reflects the conversion of shares in the current and previous years (see Note 7): Number of Restricted Shares Weighted Average Grant Date Fair Value Outstanding at January 1, 2015 481,587 $ 1.01 Granted 144,549 0.64 Vested (116,603 ) 1.24 Forfeited — — Outstanding at December 31, 2015 509,533 0.85 Granted 515,778 2.86 Vested (127,416 ) 1.69 Forfeited (3,125 ) 2.89 Outstanding at December 31, 2016 894,770 1.90 Granted 665,910 7.76 Vested (351,656 ) 1.34 Forfeited (17,680 ) 0.64 Unvested and outstanding at December 31, 2017 1,191,344 $ 4.92
Summary of Recorded Share-based Compensation Expense	Share-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands): December 31, 2017 2016 2015 Research and development $ 479 $ 152 $ 81 General and administrative 894 56 31 Total share-based compensation $ 1,373 $ 208 $ 112

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Components of Loss Before Provision for Income Taxes	The components of loss before provision for income taxes for the years ended December 31, 2017, 2016 and 2015 are as follows (in thousands): December 31, 2017 2016 2015 U.K. $ (29,978 ) $ (12,194 ) $ (13,648 ) U.S. 329 (4 ) — Loss before taxes $ (29,649 ) $ (12,198 ) $ (13,648 )
Schedule of Income Tax Provision	The Company did not record any income tax provision for the years ended December 31, 2016 and 2015. For the year ended December 31, 2017, the Company recorded the following income tax provision as follows (in thousands): December 31, 2017 Current expense (benefit): Federal $ 52 State — Foreign — Total current expense (benefit) 52 Deferred expense (benefit): Federal — State (15 ) Foreign — Total deferred tax expense (benefit) (15 ) Total income tax expense (benefit) $ 37
Schedule of Reconciliation of Income Tax Expense (Benefit)	A reconciliation of income tax expense (benefit) at the statutory corporate income tax rate to the income tax expense (benefit) at the Company’s effective income tax rate is as follows (in thousands): December 31, 2017 2016 2015 Tax (benefit) at statutory corporate rate $ (5,707 ) $ (2,439 ) $ (2,764 ) Research and development expense 869 835 800 Valuation allowance 4,304 1,122 806 Tranche obligation liability — — 780 Change in tax law 537 437 356 State taxes, net of federal benefit (15 ) — — Foreign rate differential 49 — — Other — 45 22 Income tax expense (benefit) $ 37 $ — $ —
Schedule of Net Deferred Tax Assets (Liabilities)	Net deferred tax assets (liabilities) recognized in the Company’s consolidated balance sheet consist of the following (in thousands): December 31, 2017 2016 2015 Deferred tax assets: Net operating loss carryforwards $ 6,549 $ 2,109 $ 1,273 Intangibles amortization 18 19 28 Tax credits 15 — Other 8 7 15 Total deferred tax assets 6,590 2,135 1,316 Deferred tax liabilities: Depreciation and amortization (52 ) (51 ) (45 ) Total deferred tax liabilities (52 ) (51 ) (45 ) Less: valuation allowance (6,523 ) (2,084 ) (1,271 ) Net deferred tax asset (liability) $ 15 $ — $ —

Nature of the Business - Additi

Nature of the Business - Additional Information (Details) - USD ($)	Oct. 02, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 15, 2017
Nature Of Business [Line Items]
Net loss		$ 29,686,000	$ 12,198,000	$ 13,648,000
Accumulated deficit		62,022,000	$ 32,336,000
Cash on hand		$ 129,400
Minimum
Nature Of Business [Line Items]
Number of months for operating plan sufficiently funded by cash resources		12 months
IPO
Nature Of Business [Line Items]
Price per share					$ 14
American Depositary Shares
Nature Of Business [Line Items]
Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses	$ 77,400,000
American Depositary Shares | IPO
Nature Of Business [Line Items]
Issuance of ordinary shares, shares	6,164,000
Price per share	$ 14
Nightstar Therapeutics Limited
Nature Of Business [Line Items]
Date of incorporation		Jul. 6, 2017
Nightstar Therapeutics Limited | American Depositary Shares
Nature Of Business [Line Items]
Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses	$ 2,900,000

Summary of Significant Accoun30

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017USD ($)Segment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	2
Foreign currency exchange loss	$ 2,900,000	$ 36,000	$ 58,000
Accrued interest	0	0
Accrued penalties	0	0
Gain (loss) related to foreign currency translations	$ 3,988,000	$ (1,385,000)	$ (541,000)
Emerging growth company, description	The Company would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, has more than $700 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months, and has filed one annual report on Form 20-F), or it issues more than $1 billion of non-convertible debt securities over a three-year period.
Emerging growth company status minimum annual revenue	$ 1,070,000,000
Emerging growth company status minimum market value of common stock held by non-affiliates	700,000,000
Emerging growth company status minimum issuance value of non-convertible debt securities	$ 1,000,000,000
Emerging growth company status period for issuance of non-convertible debt securities	3 years
Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of income tax realized upon settlement	50.00%
Lab Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years	3 years
Computer Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years	3 years
Office Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years	3 years

Summary of Significant Accoun31

Summary of Significant Accounting Policies - Schedule of Anti-Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unvested Restricted Share Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	1,191,344	894,770	509,533

Fair Value of Financial Asset32

Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Tranche Obligation Liability Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Details) - Fair Value Recurring Basis - Fair Value Inputs Level3 - Tranche Obligations $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	$ 3,883
Change in fair value of tranche obligations	4,333
Reclassification of tranche obligation liability to additional paid-in capital	$ (8,216)

Fair Value of Financial Asset33

Fair Value of Financial Assets and Liabilities - Additional Information (Details) - Tranche Obligations shares in Millions, $ in Millions	1 Months Ended	9 Months Ended
	Nov. 30, 2015USD ($)	Sep. 30, 2016	Nov. 30, 2015£ / sharesshares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions, risk-free interest rate		0.42%
Probability of closing tranche		80.00%
Class A Ordinary Shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Shares issued, price per share			£ 1
Fair value of share per share price			£ 1.44
Shares, issued | shares			2.5
Other Expense
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in fair value | $	$ 4.3

Prepaid Expenses and Other Cu34

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepayments	$ 2,586	$ 877
Research and development tax credit	1,753	2,811
Value-added tax receivable	905	327
Interest receivable	98
Other	96	95
Total prepaid expenses and other current assets	$ 5,438	$ 4,110

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Less: accumulated depreciation	$ (390)	$ (160)
Total property and equipment, net	355	363
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	363	212
Computer and Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 382	$ 311

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation	$ 206,000	$ 138,000	$ 40,000

Accrued Expenses and Other Li37

Accrued Expenses and Other Liabilities - Components of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Research and development activities	$ 3,779	$ 3,692
Compensation and benefits	1,093	376
Professional fees	410	129
Other	907	125
Total accrued expenses and other liabilities	$ 6,189	$ 4,322

Shareholders' Equity - Addition

Shareholders' Equity - Additional Information (Details)

Oct. 02, 2017USD ($)$ / sharesshares

Sep. 15, 2017$ / sharesshares

Jun. 27, 2017USD ($)shares

Dec. 31, 2014USD ($)

Jun. 30, 2017USD ($)$ / sharesshares

Sep. 30, 2016USD ($)shares

Nov. 30, 2015USD ($)Patientshares

Jun. 30, 2015USD ($)shares

May 31, 2014USD ($)shares

Nov. 30, 2013USD ($)shares

Dec. 31, 2017USD ($)Vote$ / shares

Dec. 31, 2017£ / sharesshares

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)shares

Dec. 31, 2018shares

Sep. 12, 2017£ / shares

Sep. 11, 2017shares

Aug. 31, 2017shares

Mar. 31, 2017£ / sharesshares

Dec. 31, 2016£ / sharesshares

Nov. 30, 2015£ / shares

Jun. 30, 2015£ / shares

May 31, 2014£ / shares

Class Of Stock [Line Items]

Number of votes per ordinary shares | Vote

1

Dividends | $

$ 0

Ordinary shares, authorized

100,000,000

70,000,000

Ordinary shares, nominal value | £ / shares

£ 0.01

£ 0.01

£ 0.01

Ordinary shares, outstanding

28,904,714

8,693,625

12,545,314

Description of Distribution of assets on liquidation or return of capital

Under the amended and restated Articles of Association in effect at such time, in the event of a distribution of assets on liquidation or a return of capital, the surplus remaining after payment of the Company’s liabilities was to be applied first in paying to each holder of Class A ordinary shares an amount per share held equal to £1.00; second in paying to each holder of Class A, B and C ordinary shares an amount per share held equal to £0.40; third in paying to each holder of Class A, B, C and D ordinary shares an amount per share equal to £0.67366; fourth in paying to each holder of Class A, B, C, D and E ordinary shares an amount per share to the difference between $4.01648 per share and £2.07366 (calculated based on the exchange rate in effect as of the date of determination); and fifth in paying to each holder of Class A, B, C, D, E and F ordinary shares and remaining amounts on a pro rata basis.

Deferred shares, per share | £ / shares

£ 1

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 141,093,000

$ 11,317,000

$ 13,959,000

Shares issued reminder price per share | £ / shares

£ 2.07366

Issuance of shares

28,904,714

8,693,625

12,545,314

Estimated fair value of tranche obligation liability | $

$ 4,200,000

Change in fair value of tranche obligation liability | $

$ 3,900,000

$ (4,300,000)

$ 4,333,000

Tranche Obligations

Class Of Stock [Line Items]

Ordinary shares, outstanding

0

Tranche Obligations | Scenario Forecast

Class Of Stock [Line Items]

Ordinary shares, outstanding

0

Tranche 2

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

9,225,139

Amended 2013 Agreement | Tranche 3A and Tranche 3B

Class Of Stock [Line Items]

Price per share | £ / shares

1

Amended 2013 Agreement | Tranche 1

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

2,500,000

Amended 2013 Agreement | Tranche 2

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

6,500,000

March 2017 Agreement

Class Of Stock [Line Items]

Termination date

2017-06

NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, nominal value | £ / shares

£ 0.00001

IPO

Class Of Stock [Line Items]

Price per share | $ / shares

$ 14

Ordinary shares, outstanding

22,740,714

American Depositary Shares

Class Of Stock [Line Items]

Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses | $

$ 77,400,000

American Depositary Shares | Nightstar Therapeutics Limited

Class Of Stock [Line Items]

Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses | $

$ 2,900,000

American Depositary Shares | IPO

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

6,164,000

Price per share | $ / shares

$ 14

Class A Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

7,941,989

11,277,900

Distribution of assets on liquidation or a return of capital per share | £ / shares

£ 1

Issuance of shares

7,941,989

11,277,900

Class A Ordinary Shares | Tranche 3

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

2,725,139

Price per share | £ / shares

2.07366

Class A Ordinary Shares | Tranche 4

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

2,725,139

Price per share | £ / shares

2.07366

Class A Ordinary Shares | 2013 Agreement

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

1,000,000

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 1,600,000

Equity issuance costs | $

$ 22,000

Class A Ordinary Shares | Amended 2013 Agreement | Tranche 2A

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

6,000,000

Price per share | £ / shares

£ 1

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 10,100,000

Class A Ordinary Shares | Amended 2013 Agreement | Tranche 2B

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

1,000,000

Price per share | £ / shares

£ 1

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 1,600,000

Class A Ordinary Shares | Amended 2013 Agreement | Tranche 3A and Tranche 3B

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

9,000,000

Price per share | £ / shares

1

Company commitment description

Lastly, the Company committed to issuing additional shares upon the completion of both (i) one or more patient studies that enrolled more than 15 patients, or such lesser amount may be agreed, with a one-year efficacy read-out that the investor determined justified the initiation of a pivotal trial and (ii) release of NSR-REP1 that the investor determined met the standard of commercial Good Manufacturing Practice (“cGMP”) as determined by the FDA and EMA (“Tranche 3A”). Further, shares could be issued in a final tranche upon the completion of a material event in the business plan for the Company for the period from completion of the Tranche 3A milestones until 12 months after the Company receives approval for NSR-REP1 in the United States (“Tranche 3B”).

Adjusted shares issued, price per share | £ / shares

£ 1.40

Class A Ordinary Shares | Amended 2013 Agreement | Tranche 3A and Tranche 3B | Minimum

Class Of Stock [Line Items]

Number of patients | Patient

15

Class A Ordinary Shares | 2015 Agreement

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

6,605,103

5,225,139

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 11,300,000

$ 12,500,000

Equity issuance costs | $

$ 139,000

Class A Ordinary Shares | March 2017 Agreement

Class Of Stock [Line Items]

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 18,800,000

Equity issuance costs | $

$ 50,000

Subscription of aggregate shares

8,070,314

Issuance of shares

8,070,314

Class A Ordinary Shares | March 2017 Agreement | Common Stock Shares Subscriptions Price One

Class Of Stock [Line Items]

Subscription of aggregate shares

1,846,071

Shares issued, price per share | £ / shares

£ 1

Class A Ordinary Shares | March 2017 Agreement | Common Stock Shares Subscriptions Price Two

Class Of Stock [Line Items]

Subscription of aggregate shares

6,224,243

Shares issued, price per share | £ / shares

£ 2.0737

Class A Ordinary Shares | June 2017 Agreement

Class Of Stock [Line Items]

Issuance of ordinary shares, shares

11,203,837

Price per share | $ / shares

$ 4.01648

Proceeds of issuance of ordinary shares, net of issuance costs | $

$ 45,000,000

Stock issuance costs | $

$ 295,000

Class A Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, authorized

50,000,000

50,000,000

Ordinary shares, outstanding

41,604,393

Ordinary shares converted

0.5051

Class B Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

57,127

57,127

Issuance of shares

57,127

57,127

Class B Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, authorized

5,000,000

5,000,000

Ordinary shares, outstanding

138,596

Ordinary shares converted

0.4122

Class C Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

368,821

368,821

Issuance of shares

368,821

368,821

Class C Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, authorized

5,000,000

5,000,000

Ordinary shares, outstanding

699,282

Ordinary shares converted

0.4122

Class D Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

199,560

199,560

Issuance of shares

199,560

199,560

Class D Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, authorized

5,000,000

5,000,000

Ordinary shares, outstanding

699,802

Ordinary shares converted

0.3751

Class E1 Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

209,064

Issuance of shares

209,064

Class E1 Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, outstanding

969,520

Ordinary shares converted

0.3125

Class E2 Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

123,279

Issuance of shares

123,279

Class E2 Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, outstanding

481,500

Ordinary shares converted

0.3125

Class E3 Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

90,155

Issuance of shares

90,155

Class E3 Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, outstanding

464,500

Ordinary shares converted

0.3125

Class E4 Ordinary Shares

Class Of Stock [Line Items]

Ordinary shares, outstanding

90,155

Issuance of shares

90,155

Class E4 Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, outstanding

375,500

Ordinary shares converted

0.3125

Class F Ordinary Shares

Class Of Stock [Line Items]

Issuance of shares

369,000

Class F Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, authorized

5,000,000

5,000,000

Ordinary shares, outstanding

1,854,780

Ordinary shares converted

0.2182

Class E Ordinary Shares | NightstaRx Limited

Class Of Stock [Line Items]

Ordinary shares, authorized

5,000,000

5,000,000

Class A, B and C Ordinary Shares

Class Of Stock [Line Items]

Distribution of assets on liquidation or a return of capital per share | £ / shares

0.40

Class A, B, C and D Ordinary Shares

Class Of Stock [Line Items]

Distribution of assets on liquidation or a return of capital per share | £ / shares

0.67366

Class A, B, C, D and E Ordinary Shares

Class Of Stock [Line Items]

Distribution of assets on liquidation or a return of capital per share | (per share)

$ 4.01648

£ 2.07366

Shareholders' Equity - Summary

Shareholders' Equity - Summary of Number of Ordinary Shares Issued and Outstanding (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Ordinary shares, issued	28,904,714	12,545,314	8,693,625
Total ordinary shares, outstanding	28,904,714	12,545,314	8,693,625
Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued	28,904,714
Total ordinary shares, outstanding	28,904,714
Class A Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		11,277,900	7,941,989
Total ordinary shares, outstanding		11,277,900	7,941,989
Class B Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		57,127	57,127
Total ordinary shares, outstanding		57,127	57,127
Class C Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		368,821	368,821
Total ordinary shares, outstanding		368,821	368,821
Class D Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		199,560	199,560
Total ordinary shares, outstanding		199,560	199,560
Class E1 Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		209,064
Total ordinary shares, outstanding		209,064
Class E2 Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		123,279
Total ordinary shares, outstanding		123,279
Class E3 Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		90,155
Total ordinary shares, outstanding		90,155
Class E4 Ordinary Shares
Class Of Stock [Line Items]
Ordinary shares, issued		90,155
Total ordinary shares, outstanding		90,155
Deferred Shares
Class Of Stock [Line Items]
Ordinary shares, issued		129,253	126,128
Total ordinary shares, outstanding		129,253	126,128

Shareholders' Equity - Summar40

Shareholders' Equity - Summary of Remaining Outstanding Tranche Obligations and Respective Prespecified Milestone Criteria (Details) - £ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015
Class Of Stock [Line Items]
Number of Shares	12,545,314	28,904,714	8,693,625
Remaining Outstanding Obligations Tranche 2
Class Of Stock [Line Items]
Number of Shares	1,846,070
Price Per Share	£ 1
Milestone	Commencement of a pivotal trial for CHM
Remaining Outstanding Obligations Tranche 2
Class Of Stock [Line Items]
Number of Shares	773,966
Price Per Share	£ 2.07366
Milestone	Commencement of a pivotal trial for CHM
Remaining Outstanding Obligations Tranche 3
Class Of Stock [Line Items]
Number of Shares	2,725,139
Price Per Share	£ 2.07366
Milestone	First patient treated in a clinical trial for a second program
Remaining Outstanding Obligations Tranche 4
Class Of Stock [Line Items]
Number of Shares	2,725,139
Price Per Share	£ 2.07366
Milestone	First patient treated in a clinical trial for a third program

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Details)	Jan. 01, 2018shares	Aug. 31, 2017EmployeeDirector£ / sharesshares	Jul. 31, 2017Employee$ / sharesshares	Jun. 30, 2017	Dec. 31, 2017USD ($)Class$ / sharesshares	Dec. 31, 2016USD ($)Classshares	Dec. 31, 2015USD ($)shares	Dec. 31, 2017£ / shares	Sep. 30, 2017shares	Sep. 12, 2017£ / shares	Dec. 31, 2016£ / shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Repurchase of deferred shares, price per share | £ / shares								£ 0.01
Weighted average grant-date fair value of share options granted | $ / shares					$ 11.56
Share compensation expense | $					$ 1,373,000	$ 208,000	$ 112,000
Impact of modification to vesting commencement dates on share-based compensation expense | $					$ 500,000
Ordinary shares, issued					28,904,714	12,545,314	8,693,625
Ordinary shares, nominal value | £ / shares								£ 0.01		£ 0.01	£ 0.01
Class C Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted							12,000
Ordinary shares, issued						368,821	368,821
Class D Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted							133,000
Vesting commencement date				May 1, 2015
Ordinary shares, issued						199,560	199,560
Class E Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of classes of ordinary shares | Class					4	4
Class E1 Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					94,000	212,000
Ordinary shares, issued						209,064
Class E2 Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					27,000	124,000
Vesting commencement date				Jun. 6, 2017
Ordinary shares, issued						123,279
Class E3 Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					55,000	90,000
Vesting commencement date				Apr. 4, 2017
Ordinary shares, issued						90,155
Class E4 Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					27,000	90,000
Vesting commencement date				Jun. 6, 2017
Ordinary shares, issued						90,155
Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					405,000
Ordinary shares, issued		369,000
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Granted					191,275	0	0
Unvested Restricted Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation, award vesting period					4 years
Share-based compensation arrangement award vesting rights description					the balance vesting periodically over the remaining three years
Share-based compensation award remaining vesting period					3 years
Restricted shares granted					665,910	515,778	144,549
Unrecognized compensation cost related to unvested restricted shares outstanding | $					$ 5,000,000	$ 1,700,000	$ 391,000
Weighted-average period of cost expected to be recognized					3 years 3 months 18 days	4 years 4 months 24 days	4 years
Unvested Restricted Shares | Share-Based Compensation Award, Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation percentage of award vesting rights					25.00%
Performance Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation, award vesting period					4 years
Share-based compensation arrangement award vesting rights description					the balance vesting periodically over the remaining three years
Share-based compensation award remaining vesting period					3 years
Performance Awards | Share-Based Compensation Award, Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation percentage of award vesting rights					25.00%
Two Thousand Fifteen Performance Based Restricted Employee Incentive Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share compensation expense | $					$ 8,000	$ 13,000	$ 11,000
Two Thousand Sixteen Performance Based Restricted Employee Incentive Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share compensation expense | $					242,000	$ 106,000
2017 Performance and Service Based Restricted Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share compensation expense | $					$ 58,000
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					58,000
Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted					666,000	516,000	145,000
Employee | Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares, issued		309,000	36,000
Number of employees | Employee		5	2
Ordinary shares, nominal value | (per share)		£ 0.00001	$ 0.00001
Chief Executive Officer | Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares, issued		133,000
Chief Development Officer | Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares, issued		130,000
Chief Scientific Officer | Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares, issued		22,000
Chief Financial Officer | Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares, issued		21,000
Board of Directors | Class F Ordinary Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares, issued		60,000
Ordinary shares, nominal value | £ / shares		£ 0.00001
Number of members of board of directors | Director		3
2017 Equity Incentive Plan | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of ordinary shares added under share-based compensation plan	1,156,188
2017 Equity Incentive Plan | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of ordinary shares authorized under share-based compensation plan									1,500,000

Share-Based Compensation - Assu

Share-Based Compensation - Assumptions Used in Black-Scholes Option Pricing Model To Determine Fair Value Of Share Options Granted (Details) - Employees and Directors	12 Months Ended
	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected option life (years)	6 years
Risk-free interest rate, minimum	2.16%
Risk-free interest rate, maximum	2.25%
Expected volatility, minimum	57.17%
Expected volatility, maximum	57.43%
Expected dividend yield	0.00%

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Share Option Activity under the 2017 Equity Incentive Plan (Details) - 2017 Equity Incentive Plan - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Options
Number of Options, Granted	191,275	0	0
Number of options, Outstanding at December 31, 2017	191,275
Number of options, Vested and expected to vest as of December 31, 2017	191,275
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Granted	$ 21
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2017	21
Weighted Average Grant Date Fair Value, Vested and expected to vest as of December 31, 2017	$ 21
Weighted Average Remaining Contractual Life
Weighted Average Remaining Contractual Life, Outstanding at December 31, 2017	9 years 10 months 13 days
Weighted Average Remaining Contractual Life, Vested and expected to vest as of December 31, 2017	9 years 10 months 13 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Granted	$ 0.64
Aggregate Intrinsic Value, Exercised	0
Aggregate Intrinsic Value, Outstanding at December 31, 2017	$ 21

Share-Based Compensation - Su44

Share-Based Compensation - Summary of Changes in the Company's Restricted Ordinary Shares (Details) - Unvested Restricted Shares - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Restricted Shares
Beginning balance	894,770	509,533	481,587
Granted	665,910	515,778	144,549
Vested	(351,656)	(127,416)	(116,603)
Forfeited	(17,680)	(3,125)
Ending balance	1,191,344	894,770	509,533
Weighted Average Grant Date Fair Value
Beginning balance	$ 1.90	$ 0.85	$ 1.01
Granted	7.76	2.86	0.64
Vested	1.34	1.69	1.24
Forfeited	0.64	2.89
Ending balance	$ 4.92	$ 1.90	$ 0.85

Share-Based Compensation - Su45

Share-Based Compensation - Summary of Recorded Share-based Compensation Expense (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total share-based compensation	$ 1,373,000	$ 208,000	$ 112,000
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total share-based compensation	479,000	152,000	81,000
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total share-based compensation	$ 894,000	$ 56,000	$ 31,000

License Agreements - Additional

License Agreements - Additional Information (Details)	1 Months Ended						12 Months Ended
	Oct. 31, 2017USD ($)	Oct. 31, 2017GBP (£)	Nov. 30, 2015USD ($)AgreementDisease	Dec. 31, 2013USD ($)	Nov. 30, 2013USD ($)	Nov. 30, 2013GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
License Agreements [Line Items]
Research and development							$ 20,502,000		$ 10,165,000	$ 7,175,000
2013 Oxford Agreement
License Agreements [Line Items]
License agreement entered date							2013-11	2013-11
Upfront fees					$ 78,000
Past patent cost					68,000
Research and development							$ 0		0	0
2013 Oxford Agreement | Maximum [Member]
License Agreements [Line Items]
Milestone fees payments upon achievement of specified sales					$ 507,000	£ 375,000
2015 Oxford Agreements
License Agreements [Line Items]
License agreement entered date							2015-11	2015-11
Upfront fees			$ 575,000
Past patent cost			$ 2,000
Number of separate exclusive license agreements | Agreement			5
Number of different types of inherited retinal diseases | Disease			5
Annual licenses maintenance fee							$ 68,000	£ 50,000
Increase of annual licenses maintenance fee, through 2025 and beyond							135,000	£ 100,000
Research and development							$ 0		0	577,000
2017 Oxford Agreements
License Agreements [Line Items]
License agreement entered date							2017-10	2017-10
Upfront fees	$ 135,000	£ 100,000
Annual licenses maintenance fee | £								£ 0
Increase of annual licenses maintenance fee in 2018 | £								10,000
2017 Oxford Agreements | Maximum [Member]
License Agreements [Line Items]
Increase of annual licenses maintenance fee, through 2025 and beyond | £								£ 20,000
Oxford BioMedica License
License Agreements [Line Items]
License agreement entered date							2013-12	2013-12
Upfront fees				$ 100,000
Research and development							$ 0		$ 0	$ 0
Milestone payment upon approval of a product				$ 100,000

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Loss Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Loss before taxes	$ (29,649)	$ (12,198)	$ (13,648)
U.K.
Income Tax Disclosure [Line Items]
Loss before taxes	(29,978)	(12,194)	$ (13,648)
U.S.
Income Tax Disclosure [Line Items]
Loss before taxes	$ 329	$ (4)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017USD ($)TaxReturnExamination	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Jan. 01, 2018
Income Tax Disclosure [Line Items]
Income tax provision	$ 37,000	$ 0	$ 0
Tax relief percentage of allowable research and development costs	11.00%
Unrecognized tax benefits	$ 0	0	0
Income tax examination, description	The income tax returns are generally subject to tax examinations for the tax years ended December 31, 2013 through December 31, 2017.
Number of pending income tax return examinations | TaxReturnExamination	0
Subsequent Event
Income Tax Disclosure [Line Items]
Tax relief percentage of allowable research and development costs				12.00%
U.K.
Income Tax Disclosure [Line Items]
Maximum parentage of tax credit payable for eligible research and development expenditures	33.40%
Maximum percentage of cash rebate eligible by subcontracted research expenditures	21.70%
Research and development foreign tax credit used offset research and development expense	$ 1,300,000	2,200,000	2,500,000
Gross U.K. net operating loss carryforwards	$ 37,900,000	$ 12,400,000	$ 7,100,000

Income Taxes - Schedule of Inco

Income Taxes - Schedule of Income Tax Provision (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current expense (benefit):
Federal	$ 52,000
Total current expense (benefit)	52,000
Deferred expense (benefit):
State	(15,000)
Total deferred tax expense (benefit)	(15,000)
Total income tax expense (benefit)	$ 37,000	$ 0	$ 0

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax Expense (Benefit) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax (benefit) at statutory corporate rate	$ (5,707,000)	$ (2,439,000)	$ (2,764,000)
Research and development expense	869,000	835,000	800,000
Valuation allowance	4,304,000	1,122,000	806,000
Tranche obligation liability			780,000
Change in tax law	537,000	437,000	356,000
State taxes, net of federal benefit	(15,000)
Foreign rate differential	49,000
Other		45,000	22,000
Total income tax expense (benefit)	$ 37,000	$ 0	$ 0

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 6,549	$ 2,109	$ 1,273
Intangibles amortization	18	19	28
Tax credits	15
Other	8	7	15
Total deferred tax assets	6,590	2,135	1,316
Deferred tax liabilities:
Depreciation and amortization	(52)	(51)	(45)
Total deferred tax liabilities	(52)	(51)	(45)
Less: valuation allowance	(6,523)	$ (2,084)	$ (1,271)
Net deferred tax asset (liability)	$ 15

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2017EUR (€)
Commitments And Contingencies [Line Items]
Litigation contingency reserves liability	$ 0	$ 0
Operating lease, aggregate lease payments, 2018	86,000
Operating lease, aggregate lease payments, 2019	89,000
Operating lease, aggregate lease payments, 2020	$ 46,000
Operating lease payments commencement date	Feb. 1, 2017
Operating lease terminate period	2020-06
Rent expense associated with lease	$ 205,000	138,000	$ 106,000
Credit Arrangement
Commitments And Contingencies [Line Items]
Line of credit facility, maximum borrowing capacity per month | €				€ 300,000
Outstanding credit arrangement	$ 0	$ 0

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details)	1 Months Ended			12 Months Ended
	Oct. 31, 2017USD ($)	Oct. 31, 2017GBP (£)	Sep. 30, 2017shares	Dec. 31, 2017USD ($)shares	Dec. 31, 2017GBP (£)shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Related Party Transaction [Line Items]
Research and development				$ 20,502,000		$ 10,165,000	$ 7,175,000
General and administrative				7,001,000		2,055,000	2,149,000
Consulting fees outstanding				$ 410,000		129,000
Restricted Stock Units
Related Party Transaction [Line Items]
Granted | shares				58,000	58,000
Minimum | Management Rights Letter
Related Party Transaction [Line Items]
Percentage of non controlling interest				5.00%
Syncona
Related Party Transaction [Line Items]
Professional fees				$ 1,200		23,000	52,000
Outstanding payables				1,300		0
University of Oxford and Related Entities
Related Party Transaction [Line Items]
Research and development				862		376,000	871,000
University of Oxford and Related Entities | Accrued Expenses
Related Party Transaction [Line Items]
Research and development				414,000		248,000
Freeline Therapeutics
Related Party Transaction [Line Items]
Outstanding payables				12,200		11,000
Research and development				11,000		30,000	0
The Wellcome Trust
Related Party Transaction [Line Items]
Outstanding payables				182,000		44,000
General and administrative				165,000		152,000	182,000
Oxford University Innovation Ltd | MacLaren Agreement
Related Party Transaction [Line Items]
Professional fees	$ 134,000	£ 99,000		$ 107,000		$ 150,000	$ 90,000
License agreement entered date				2013-11	2013-11
License agreement terminates date				2019-11	2019-11
Consulting fees outstanding				$ 8,000
Oxford University Innovation Ltd | MacLaren Agreement | Restricted Stock Units
Related Party Transaction [Line Items]
Granted | shares				58,000	58,000
Share-based compensation, award vesting period				4 years	4 years
Oxford University Innovation Ltd | Minimum | MacLaren Agreement
Related Party Transaction [Line Items]
Professional fees				$ 97,000	£ 72,000
Syncona Portfolio Limited | Initial Public Offering
Related Party Transaction [Line Items]
Issuance of ordinary shares, shares | shares			1,000,000
New Enterprise Associates | Initial Public Offering
Related Party Transaction [Line Items]
Issuance of ordinary shares, shares | shares			714,285

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Details) - Defined Benefit Pension Plan - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United Kingdom
Defined Contribution Plan Disclosure [Line Items]
Defined benefit contribution amount	$ 200,000	$ 200,000	$ 100,000
United States
Defined Contribution Plan Disclosure [Line Items]
Matching contributions by employer recorded amount	$ 0	$ 0	$ 0