Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (21,698)	$ (11,295)	$ (10,347)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	132	89	19
Purchase premium on marketable securities	(146)	(5)
Amortization of premium on marketable securities	101	2
Deferred rent	12	4
Amortization of lease allowance liability	(22)	(23)
Share/stock-based compensation	1,221	767	147
Change in fair value of forward sale contract			1,444
Discount on Series A convertible preferred shares			531
Research and development services settled with convertible preferred stock	110	812	845
Changes in operating assets and liabilities:
Accounts receivable - grant	482	(1,697)
Prepaid expenses and other assets	(924)	(586)	(112)
Accounts payable	(8)	(169)	(165)
Deferred revenue	71
Accrued and other liabilities	1,829	1,119	303
Net cash used in operating activities	(18,840)	(10,982)	(7,335)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(212)	(208)	(181)
Purchases of marketable securities	(20,390)	(3,766)
Proceeds from maturities of marketable securities	8,446
Decrease (increase) in restricted cash	80	(40)	(40)
Net cash used in investing activities	(12,076)	(4,014)	(221)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of convertible preferred stock, net of offering costs		43,679	5,575
Proceeds from initial public offering, net of (payments of) offering costs	49,294	(2,028)
Proceeds from employee stock plan purchases and issuance of common stock	76	23
Net cash provided by financing activities	49,370	41,674	5,575
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	18,454	26,678	(1,981)
CASH AND CASH EQUIVALENTS
Beginning of period	29,294	2,616	4,597
End of period	47,748	29,294	2,616
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to purchase of property and equipment	172
Deemed dividend to Series A convertible preferred stockholders upon conversion from an LLC to corporation		228
Convertible preferred stock issued for research and development services to be performed		232
Settlement on forward sale contract of Series A convertible preferred shares			$ 1,936
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)
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