CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (18,222)	$ (20,600)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	372	189
Share-based compensation	1,866	1,275
Realized (gain) loss on investments	(1)
Unrealized foreign currency transaction (gain) loss	(29)	256
Accretion of discount on marketable securities	(561)	(112)
Changes in operating assets and liabilities:
Other receivables	(1,697)	217
Prepaid expenses and other current assets	1,190	597
Tax incentive receivables	(235)	(428)
Other assets	(745)
Accounts payable	(2,526)	(2,747)
Accrued expenses and other current liabilities	(2,398)	810
Deferred rent		(59)
Other long-term liabilities	(8)
Net cash used in operating activities	(22,994)	(20,602)
Cash flows from investing activities:
Purchases of marketable securities	(32,853)	(22,825)
Proceeds from maturities of marketable securities	82,537	5,000
Purchases of property and equipment	(168)
Net cash provided by (used in) investing activities	49,516	(17,825)
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of commissions	10,251
Payment of offering costs	(79)	(247)
Proceeds from stock option exercises	359	51
Net cash provided by financing activities	10,531	(196)
Net increase (decrease) in cash and cash equivalents	37,053	(38,623)
Cash, cash equivalents and restricted cash at beginning of period	34,080	87,338
Cash, cash equivalents and restricted cash at end of period	71,133	48,715
Supplemental disclosure of non-cash investing and financing activities:
Offering costs included in accounts payable and accrued expenses	$ 79	$ 246