Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (20,753)	$ (16,215)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	183	96
Purchase discount (premium) on marketable securities, net	2	(146)
Amortization of premium on marketable securities, net	82	51
Stock-based compensation	1,794	963
Research and development services settled with convertible preferred stock	15	80
Other, net	(15)	(16)
Changes in operating assets and liabilities:
Accounts receivable - grant	(1,237)	126
Prepaid expenses and other assets	275	(1,261)
Accounts payable	447	347
Deferred revenue	(21)	181
Accrued and other liabilities	486	1,309
Net cash used in operating activities	(18,742)	(14,485)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(428)	(91)
Purchases of marketable securities	(62,115)	(20,390)
Proceeds from maturities of marketable securities	35,170	5,766
Decrease in restricted cash		80
Net cash used in investing activities	(27,373)	(14,635)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock in public offering, net of offering costs	11,380	49,294
Proceeds from employee stock plan purchases and stock option exercises	149	76
Net cash provided by financing activities	11,529	49,370
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(34,586)	20,250
CASH AND CASH EQUIVALENTS
Beginning of period	47,748	29,294
End of period	$ 13,162	49,544
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