Condensed Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Operating activities
Net loss	$ (15,343)	$ (9,146)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	209	228
Non-cash interest expense	18	18
Stock-based compensation expense	1,137	429
Change in fair value of warrant liability	(50)	114
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(913)	(274)
Accounts payable	1,706	(419)
Accrued expenses	468	(157)
Deferred revenue	(715)	1,950
Net cash used in operating activities	(13,483)	(7,257)
Investing activities
Purchases of property and equipment	(132)
Net cash used in investing activities	(132)
Financing activities
Proceeds from issuance of common stock in initial public offering, net of offering costs	51,505
Proceeds from issuance of convertible notes payable, net of debt issuance costs		3,500
Proceeds from issuance of notes payable, net of debt issuance costs		3,000
Payments on equipment financing and notes payable	(833)	(62)
Proceeds from exercise of common stock options and warrants	32	15
Net cash provided by financing activities	50,704	6,453
Net increase (decrease) in cash and cash equivalents	37,089	(804)
Cash and cash equivalents at beginning of period	7,942	7,882
Cash and cash equivalents at end of period	45,031	7,078
Supplemental non-cash financing activities
Conversion of preferred stock into common stock	56,678
Conversion of preferred stock warrants into common stock warrants	247
Issuance of warrants		287
Supplemental cash flow information
Cash paid for interest	$ 123	$ 120