Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (16,215)	$ (7,308)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	96	60
Purchase premium on marketable securities	(146)	(5)
Amortization of premium on marketable securities	51	1
Loss on disposal of property and equipment	1	2
Deferred rent		3
Amortization of lease allowance liability	(17)	(17)
Stock-based compensation	963	348
Research and development services settled with convertible preferred stock	80	695
Changes in operating assets and liabilities:
Accounts receivable-grant	126	(4,235)
Prepaid expenses and other assets	(1,261)	(528)
Accounts payable	347	217
Deferred revenue	181
Accrued and other liabilities	1,309	618
Net cash used in operating activities	(14,485)	(10,149)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(91)	(168)
Purchases of marketable securities	(20,390)	(3,296)
Proceeds from maturities of marketable securities	5,766
Decrease (increase) in restricted cash	80	(40)
Net cash used in investing activities	(14,635)	(3,504)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of convertible preferred stock, net of offering costs		43,679
Proceeds from initial public offering, net of (payments of) offering costs	49,294	(1,802)
Proceeds from employee stock plan purchases	76
Net cash provided by financing activities	49,370	41,877
NET INCREASE IN CASH	20,250	28,224
CASH
Beginning of period	29,294	2,616
End of period	49,544	30,840
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
Convertible preferred stock issued for research and development services to be performed		$ 349
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