STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net loss	$ (22,628)	$ (17,237)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	16	8
Stock-based compensation expense	2,276	1,368
Amortization of debt issuance costs	5	5
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,054	(1,260)
Accounts payable and accrued expenses	556	2,036
Net cash used in operating activities	(18,721)	(15,080)
Cash flows from investing activities
Purchases of investments	(2,434)	(6,961)
Maturities of short-term investments	4,225	498
Purchases of equipment	(643)	(352)
Net cash provided by (used in) investing activities	1,148	(6,815)
Cash flows from financing activities
Proceeds from exercise of stock options	123	373
Financing costs	(167)	(89)
Payments on notes payable	(1,210)	(82)
Net cash (used in) provided by financing activities	(1,254)	202
Net decrease in cash and cash equivalents	(18,827)	(21,693)
Cash and cash equivalents-beginning of period	51,722	49,720
Cash and cash equivalents-end of period	32,895	28,027
Supplemental disclosure of cash flow information
Cash paid for interest	360	348
Financing arrangement with third-party vendor		$ 584
Payment on notes payable in accounts payable	43
Purchase of property and equipment in accounts payable	$ 6