STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities
Net loss	$ (16,165)	$ (12,275)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	9	6
Stock-based compensation expense	1,476	771
Amortization of debt issuance costs	4	4
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,186	219
Accounts payable and accrued expenses	1,228	2,097
Net cash used in operating activities	(12,262)	(9,178)
Cash flows from investing activities
Purchases of investments	(2,434)	(1,740)
Purchases of equipment	(406)
Maturities of short-term investments	2,735	249
Deposit on equipment		(352)
Net cash used in investing activities	(105)	(1,843)
Cash flows from financing activities
Proceeds from exercise of stock options	123	234
Financing costs	(167)
Payments on notes payable	(209)
Net cash (used in) provided by financing activities	(253)	234
Net decrease in cash and cash equivalents	(12,620)	(10,787)
Cash and cash equivalents-beginning of period	51,722	49,720
Cash and cash equivalents-end of period	39,102	38,933
Supplemental disclosure of cash flow information
Cash paid for interest	243	$ 230
Payment on notes payable in accounts payable	43
Property and equipment in accounts payable	$ 227