Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net loss	$ (10,373)	$ (7,028)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,146	809
Depreciation and amortization	386	393
Deferred rent costs	(142)	(130)
Changes in operating assets and liabilities:
Other receivables		(480)
Income tax receivable	(50)	4
Prepaid expenses and other current assets	(303)	(561)
Accounts payable and accrued expenses	1,204	69
Deferred revenue	(1,363)	(406)
Net cash used in operating activities	(9,495)	(7,330)
Investing activities
Purchases of property and equipment	(12)	(6)
Net cash used in investing activities	(12)	(6)
Financing activities
Proceeds from initial public offering, net of issuance costs		57,762
Proceeds from the exercise of stock options	35	29
Net cash provided by financing activities	35	57,791
Net cash (decrease) increase for the period	(9,472)	50,455
Cash and cash equivalents at beginning of period	52,663	12,357
Cash and cash equivalents at end of period	$ 43,191	62,812
Noncash financing activities
Conversion of convertible preferred stock to common stock		65,586
Reclassification of prepaid IPO costs paid in 2013		1,465
Unpaid IPO issuance costs		$ 50