Condensed Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Operating activities
Net loss	$ (26,017)	$ (12,959)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	311	335
Non-cash interest expense	28	28
Stock-based compensation expense	1,945	755
Change in fair value of warrant liability	(50)	99
Change in fair value of convertible notes payable		188
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(258)	106
Accounts payable	1,692	(437)
Accrued expenses	984	(210)
Deferred revenue	(804)	1,658
Net cash used in operating activities	(22,169)	(10,437)
Investing activities
Purchases of property and equipment	(137)
Net cash used in investing activities	(137)
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	(1,249)	(83)
Proceeds from exercise of common stock options and warrants	51	19
Net cash provided by financing activities	50,307	6,436
Net increase (decrease) in cash and cash equivalents	28,001	(4,001)
Cash and cash equivalents at beginning of period	7,942	7,882
Cash and cash equivalents at end of period	35,943	3,881
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	$ 177	$ 197