Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (12,879)	$ (9,977)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	114	62
Purchase premium on marketable securities	(40)	(17)
Amortization of premium on marketable securities	80	14
Stock-based compensation	1,066	529
Research and development services settled with convertible preferred stock		50
Other, net	(13)	(12)
Changes in operating assets and liabilities:
Accounts receivable - grant	2	72
Prepaid expenses and other assets	29	(687)
Accounts payable	340	172
Accrued and other liabilities	(73)	1,174
Net cash used in operating activities	(11,374)	(8,620)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(317)	(91)
Purchases of marketable securities	(37,780)	(2,940)
Proceeds from maturities of marketable securities	17,010	3,766
Increase in restricted cash		(75)
Net cash (used in) provided by investing activities	(21,087)	660
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock in public offering, net of offering costs	11,551	49,294
Proceeds from employee stock plan purchases and stock option exercises	96
Net cash provided by financing activities	11,647	49,294
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(20,814)	41,334
CASH AND CASH EQUIVALENTS
Beginning of period	47,748	29,294
End of period	$ 26,934	70,628
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