CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (10,644)	$ (6,411)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	95	87
Change in fair value of derivative liabilities		(1,199)
Share-based compensation	614	62
Unrealized foreign currency transaction (gain) loss	8
Accretion of discount on marketable securities	(45)
Changes in operating assets and liabilities:
Other receivables	(208)	190
Prepaid expenses and other current assets	604	131
Tax incentive receivables	(45)	(159)
Accounts payable	(2,469)	(82)
Accrued expenses and other current liabilities	165	434
Deferred rent	(36)	(497)
Advance payments from collaborator		465
Net cash used in operating activities	(11,961)	(6,979)
Cash flows from investing activities:
Purchases of marketable securities	(22,825)
Net cash used in investing activities	(22,825)
Cash flows from financing activities:
Net cash provided by financing activities		43,001
Net increase in cash and cash equivalents	(34,786)	36,022
Cash, cash equivalents and restricted cash at beginning of period	87,338	10,365
Cash, cash equivalents and restricted cash at end of period	$ 52,552	46,387
Supplemental disclosure of non-cash investing and financing activities:
Conversion of bridge units into preferred units		8,500
Settlement of derivative liabilities upon issuance of preferred units		944
Cumulative dividends on redeemable convertible preferred shares		1,236
Accretion of redeemable convertible preferred units to redemption value		18
Accretion of bridge units to redemption value		576
Series C Preferred Stock [Member]
Cash flows from financing activities:
Proceeds from issuance of preferred units, net of issuance costs		$ 43,001