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	$ (27,236,000)	$ (21,698,000)	$ (11,295,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	249,000	132,000	89,000
Purchase discount (premium) on marketable securities	9,000	(146,000)	(5,000)
Amortization of premium on marketable securities	75,000	101,000	2,000
Share/stock-based compensation	2,492,000	1,221,000	767,000
Research and development services settled with convertible preferred stock	15,000	110,000	812,000
Other, net	(21,000)	(10,000)	(19,000)
Changes in operating assets and liabilities:
Accounts receivable - grant	(1,863,000)	482,000	(1,697,000)
Prepaid expenses and other assets	(114,000)	(924,000)	(586,000)
Accounts payable	164,000	(8,000)	(169,000)
Deferred revenue	(51,000)	71,000
Accrued and other liabilities	1,666,000	1,829,000	1,119,000
Net cash used in operating activities	(24,615,000)	(18,840,000)	(10,982,000)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(619,000)	(212,000)	(208,000)
Purchases of marketable securities	(64,115,000)	(20,390,000)	(3,766,000)
Proceeds from maturities of marketable securities	42,205,000	8,446,000
Decrease (increase) in restricted cash		80,000	(40,000)
Net cash used in investing activities	(22,529,000)	(12,076,000)	(4,014,000)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of convertible preferred stock, net of offering costs			43,679,000
Proceeds from issuance of common stock in public offering, net of (payments of) offering costs	11,380,000	49,294,000	(2,028,000)
Proceeds from employee stock plan purchases and stock option exercises	833,000	76,000	23,000
Net cash provided by financing activities	12,213,000	49,370,000	41,674,000
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(34,931,000)	18,454,000	26,678,000
CASH AND CASH EQUIVALENTS
Beginning of period	47,748,000	29,294,000	2,616,000
End of period	12,817,000	47,748,000	29,294,000
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to purchase of property and equipment	$ 57,000	172,000
Deemed dividend to Series A convertible preferred stockholders upon conversion from an LLC to corporation			(228,000)
Convertible preferred stock issued for research and development services to be performed			232,000
Series A Convertible Preferred Stock
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Deemed dividend to Series A convertible preferred stockholders upon conversion from an LLC to corporation			$ 228,000
Conversion of convertible preferred stock to common stock upon initial public offering		13,573,000
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		$ 44,738,000