Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (29,080)	$ (18,845)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	181	159
(Gain) loss on disposal of property and equipment	(65)	34
Amortization of premium on available-for-sale securities	272	72
Stock-based compensation	1,845	162
Change in fair value associated with redeemable convertible preferred stock tranche liability	0	4,194
Change in fair value of redeemable convertible preferred stock warrant liability	0	525
Changes in operating assets and liabilities:
Research and development tax incentive receivable	(479)	(1,030)
Prepaid expenses and other current assets	1,698	1,134
Other assets	0	(4)
Accounts payable	1,428	85
Accrued expenses and other payables	1,517	202
Net cash used in operating activities	(22,683)	(13,312)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of property and equipment	(420)	(279)
Purchase of available-for-sale securities	(8,002)	(6,396)
Proceeds from maturities of available-for-sale securities	20,265	7,788
Net cash provided by investing activities	11,843	1,113
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	0	22,508
Proceeds from issuance of common stock upon exercise of stock options and purchases under employee stock purchase plan	461	143
Payment of deferred offering costs	0	(770)
Net cash provided by financing activities	461	21,881
Effect of exchange rate changes on cash, cash equivalents and restricted cash	110	31
Net (decrease) increase in cash, cash equivalents and restricted cash	(10,269)	9,713
Cash, cash equivalents and restricted cash, beginning of period	21,094	4,065
Cash, cash equivalents and restricted cash, end of period	10,365	13,768
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION
Settlement of fair value of redeemable convertible preferred stock liability	0	5,837
Accretion of redeemable convertible preferred stock	0	265
Deferred offering costs in accounts payable and accrued liabilities	0	696
Acquisition of equipment in trade-in	185	0
Purchase of property and equipment in accounts payable and accrued liabilities	42	0
Reclassification of preferred stock warrant liability to equity	$ 0	$ 1,005