Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (17,167)	$ (8,119)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	100	70
Purchase net discount on marketable securities	243
Net accretion of discount on marketable securities	(275)	(14)
Stock-based compensation	1,081	835
Research and development services settled with stock		30
Other, net	69	(11)
Changes in operating assets and liabilities:
Accounts receivable - grant		(295)
Prepaid expenses and other assets	(928)	(102)
Accounts payable	2,289	396
Deferred revenue		(20)
Operating lease liabilities	(114)
Accrued and other liabilities	(1,105)	(217)
Net cash used in operating activities	(15,807)	(7,447)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(118)	(83)
Purchases of marketable securities	(19,750)
Proceeds from maturities of marketable securities	33,804	4,785
Net cash provided by investing activities	13,936	4,702
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock and pre-funded warrants in public offering, net of offering costs	64,662
Proceeds from employee stock plan purchases and stock option exercises	399	688
Principal payments on finance lease obligation	(6)
Net cash provided by financing activities	65,055	688
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	63,184	(2,057)
CASH AND CASH EQUIVALENTS
Beginning of period	22,461	12,817
End of period	85,645	$ 10,760
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Leased assets obtained in exchange for lease obligations	$ 776