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	$ (28,643)	$ (24,850)	$ (10,833)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	23	11	5
Stock-based compensation expense	3,077	2,108	698
Change in fair value of warrant liability			(1,192)
Amortization of debt issuance costs	7	7	4
Loss on disposal of fixed assets		2
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,396	(947)	1,344
Accounts payable and accrued expenses	(629)	3,540	1,385
Net cash used in operating activities	(24,769)	(20,129)	(8,589)
Cash flows from investing activities
Purchases of investments	(2,434)	(7,705)
Maturities of short-term investments	4,474	1,743
Purchases of property and equipment	(644)	(352)	(33)
Net cash provided by (used in) investing activities	1,396	(6,314)	(33)
Cash flows from financing activities
Proceeds from exercise of stock options	124	373	128
Proceeds from public offerings, net of offering costs	(167)	28,278	41,202
Proceeds from notes payable, net of issuance costs			6,975
Principal payments of notes payable	(2,128)	(206)
Net cash (used in) provided by financing activities	(2,171)	28,445	48,305
Net (decrease) increase in cash and cash equivalents	(25,544)	2,002	39,683
Cash and cash equivalents-beginning of year	51,722	49,720	10,037
Cash and cash equivalents-end of year	26,178	51,722	49,720
Supplemental disclosure of cash flow information
Conversion of preferred stock to common stock			70,340
Issuance of Series C Preferred Stock			500
Cash paid for interest	460	468	$ 89
Financing arrangement with third-party vendor		584
Purchase of property and equipment in accounts payable	$ 134
Accrued public offering costs		$ 142