Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (44,348)	$ (27,236)	$ (21,698)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	293	249	132
Purchase net discount (premium) on marketable securities	907	9	(146)
Net (accretion of discount) amortization of premium on marketable securities	(593)	75	101
Stock-based compensation	4,280	2,492	1,221
Research and development services settled with stock	107	15	110
Other, net	(40)	(21)	(10)
Changes in operating assets and liabilities:
Accounts receivable - grant	3,078	(1,863)	482
Prepaid expenses and other assets	(261)	(114)	(924)
Accounts payable	239	164	(8)
Deferred revenue	(20)	(51)	71
Accrued and other liabilities	4,165	1,666	1,829
Net cash used in operating activities	(32,193)	(24,615)	(18,840)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(422)	(619)	(212)
Purchases of marketable securities	(62,179)	(64,115)	(20,390)
Proceeds from maturities of marketable securities	47,370	42,205	8,446
Decrease in restricted cash			80
Net cash used in investing activities	(15,231)	(22,529)	(12,076)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock in public and at-the-market offerings, net of offering costs	54,048	11,380	49,294
Proceeds from employee stock plan purchases and stock option exercises	3,020	833	76
Net cash provided by financing activities	57,068	12,213	49,370
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	9,644	(34,931)	18,454
CASH AND CASH EQUIVALENTS
Beginning of period	12,817	47,748	29,294
End of period	22,461	12,817	47,748
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to purchase of property and equipment	$ 92	$ 57	172
Series A Convertible Preferred Stock
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Conversion of convertible preferred stock to common stock upon initial public offering			13,573
Series B Convertible Preferred Stock
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Conversion of convertible preferred stock to common stock upon initial public offering			$ 44,738