CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (39,886)	$ (32,641)	$ (13,153)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash research and development expense		4,595	3,517
Depreciation and amortization	363	279	11
Change in fair value of derivative liabilities	(1,541)	(580)	(174)
Share-based compensation	1,427	180	21
Unrealized foreign currency transaction loss	83
Changes in operating assets and liabilities:
Other receivables	(707)	(294)	(10)
Prepaid expenses and other current assets	(575)	(966)	(280)
Tax incentive receivables	(1,811)	(144)
Deposits		(53)	(150)
Accounts payable	2,349	(644)	671
Accrued expenses and other current liabilities	1,315	2,322	409
Deferred rent	(128)	(84)
Advance payments from collaborator		(929)	(470)
Net cash used in operating activities	(39,111)	(28,959)	(9,608)
Cash flows from investing activities:
Purchases of property and equipment	(27)	(830)	(232)
Net cash used in investing activities	(27)	(830)	(232)
Cash flows from financing activities:
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	77,749
Payment of initial public offering costs	(3,574)
Changes in restricted cash			(30)
Cash payment for non-controlling interests	(1,175)
Net cash provided by financing activities	116,111	34,413	15,275
Net increase in cash and cash equivalents	76,973	4,624	5,435
Cash and cash equivalents at beginning of period	10,315	5,691	256
Cash and cash equivalents at end of period	87,288	10,315	5,691
Supplemental disclosure of non-cash investing and financing activities:
Conversion of bridge units into preferred units	8,500		8,000
Conversion of preferred stock to common stock	107,018
Settlement of derivative liabilities upon issuance of preferred units	944		888
Issuance of tranche rights with preferred units		909	2,401
Deemed contribution of capital		2,408	1,419
Settlement of derivative liability upon issuance of bridge units		305
Issuance of contingent prepayment option with bridge units		908
Cumulative dividends on redeemable convertible preferred shares	6,146	3,441	932
Accretion of redeemable convertible preferred units and stock to redemption value	632	969	34
Accretion of bridge units to redemption value	576	27	2,307
Issuance of additional shares of common stock to minority investors under anti-dilution rights		980	1,459
Purchases of property and equipment included in accounts payable, accrued expenses and deferred rent			728
Deferred offering costs included in accounts payable and accrued expenses			11
Series A Preferred Stock [Member]
Cash flows from financing activities:
Proceeds from issuance of preferred units, net of issuance costs			7,330
Bridge Units [Member]
Cash flows from financing activities:
Proceeds from issuance of bridge units		8,500	8,000
Series B Preferred Stock [Member]
Cash flows from financing activities:
Payment of initial public offering costs			$ (25)
Proceeds from issuance of preferred units, net of issuance costs		$ 25,913
Series C Preferred Stock [Member]
Cash flows from financing activities:
Payment of initial public offering costs	(176)
Proceeds from issuance of preferred units, net of issuance costs	$ 43,111