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	$ (35,186)	$ (17,533)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	207	145
Purchase net discount on marketable securities	480	401
Net accretion of discount on marketable securities	(543)	(107)
Stock-based compensation	2,326	1,831
Research and development services settled with stock		41
Non-cash operating lease expense	191
Other, net		(20)
Changes in operating assets and liabilities:
Accounts receivable - grant		(1,203)
Prepaid expenses and other assets	(2,223)	(573)
Accounts payable	2,141	579
Deferred revenue		(20)
Operating lease liabilities	(178)
Accrued and other liabilities	595	(32)
Net cash used in operating activities	(32,190)	(16,491)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(175)	(118)
Purchases of marketable securities	(77,956)	(26,250)
Proceeds from maturities of marketable securities	50,054	11,870
Net cash used in investing activities	(28,077)	(14,498)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock and pre-funded warrants in public offering, net of offering costs	64,298	37,690
Proceeds from employee stock plan purchases and stock option exercises	450	1,061
Principal payments on finance lease obligation	(12)
Net cash provided by financing activities	64,736	38,751
NET INCREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	4,469	7,762
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH
Beginning of period	22,461	12,817
End of period	26,930	$ 20,579
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Leased assets obtained in exchange for lease obligations	$ 5,294