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	$ (36,957,000)	$ (37,177,000)	$ (14,858,000)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation	4,241,000	2,130,000	99,000
Net amortization of premium on available-for-sale securities	687,000	117,000	(8,000)
Depreciation and amortization	406,000	317,000	247,000
(Gain) loss on disposal of property and equipment	(62,000)	34,000
Change in fair value associated with redeemable convertible preferred stock tranche liability		4,194,000	626,000
Change in fair value of redeemable convertible preferred stock warrant liability	0	525,000	(543,000)
Changes in operating assets and liabilities:
Research and development tax credit receivable	1,070,000	(1,588,000)	(192,000)
Receivable from collaboration partner - related party	(1,816,000)
Prepaid expenses and other assets	(333,000)	(1,804,000)	(1,532,000)
Accounts payable	91,000	(115,000)	898,000
Accrued expenses and other payables	4,793,000	3,395,000	878,000
Deferred revenue - related party	31,752,000
Net cash provided by (used in) operating activities	3,872,000	(29,972,000)	(14,385,000)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of available-for-sale securities	(39,546,000)	(73,169,000)	(7,865,000)
Proceeds from maturities of available-for-sale securities	56,035,000	14,188,000
Purchase of property and equipment, net	(666,000)	(347,000)	(399,000)
Net cash provided by (used in) investing activities	15,823,000	(59,328,000)	(8,264,000)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock, net of issuance costs	64,547,000	83,648,000
Proceeds from issuance of common stock upon exercise of stock options and purchases under employee stock purchase plan	1,007,000	151,000	57,000
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		22,508,000	17,362,000
Net cash provided by financing activities	65,554,000	106,307,000	17,419,000
Effect of exchange rate changes on cash, cash equivalents and restricted cash	146,000	22,000	(39,000)
Net increase (decrease) in cash, cash equivalents and restricted cash	85,395,000	17,029,000	(5,269,000)
Cash, cash equivalents and restricted cash, beginning of year	21,094,000	4,065,000	9,334,000
Cash, cash equivalents and restricted cash, end of year	106,489,000	21,094,000	4,065,000
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING INFORMATION:
Acquisition of new equipment upon trade-in for existing equipment	185,000
Deferred offering costs in accounts payable and accrued liabilities	66,000
Conversion of redeemable convertible preferred stock to common stock at closing of initial public offering		66,904,000
Settlement of fair value of redeemable convertible preferred stock liability		5,837,000
Reclassification of preferred stock warrant liability to equity		1,005,000
Accretion of redeemable convertible preferred stock	$ 0	558,000	75,000
Purchase of property and equipment in accounts payable		$ 21,000
Tranche liability in connection with the Series C redeemable convertible preferred stock financing			$ 1,017,000