Unaudited Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (4,869)	$ (7,184)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1	3
Stock-based compensation expense	396	274
Write off of deferred offering costs	0	111
Amortization of investment premium	100	0
Write off of deferred offering costs	100	0
Warrant liability fair value adjustment	(1,523)	0
Changes in deferred rent	(5)	0
Changes in operating assets and liabilities:
Prepaid expenses, other assets, and deferred costs	(549)	(477)
Accounts payable and accrued expenses	(359)	186
Accrued severance and retention cost obligations	0	(1,809)
Deferred revenue	(64)	(64)
Net cash used in operating activities	(6,772)	(8,960)
Cash flows from investing activities:
Maturities of investments	6,000	0
Purchases of property and equipment	(2)	(14)
Purchase of investments	(18,161)	0
Net cash used in investing activities	(12,163)	(14)
Cash flows from financing activities:
Proceeds from common stock issued	3,277	0
Payments of deferred costs and underwriting discounts and commissions	(108)	0
Proceeds from employee stock purchase plan issuance	18	5
Net cash provided by financing activities	3,187	5
Net decrease in cash and cash equivalents	(15,748)	(8,969)
Cash and cash equivalents, beginning of period	35,656	46,985
Cash and cash equivalents, end of period	19,908	38,016
Supplemental cash flow information:
Cash received for interest	345	0
Cash received for interest	60	28
Noncash financing and investing activities:
Deferred offering costs included in accounts payable and accrued expenses	0	25
Deferred offerings cost reclassed to additional paid in capital	$ 10	$ 0