Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (40,022)	$ (27,097)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	78	33
Amortization of premium (accretion of discount) on marketable securities, net	682
Stock-based compensation expense	5,850	1,815
Research and development expense settled in stock		2,717
Change in fair value of other current assets, warrant liability and other liability		505
Non-cash interest	156	127
Loss on retirement of note payable		57
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,006)	(1,420)
Other long-term assets	(30)	(150)
Accounts payable	999	888
Accrued expenses and other current liabilities	(5,906)	3,254
Net cash used in operating activities	(39,199)	(19,271)
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of property and equipment	(186)	(29)
Purchases of marketable securities	(179,338)	(30,649)
Sales and maturities of marketable securities	75,802
Net cash provided by (used in) investing activities	(103,722)	(30,678)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from note payable, net		20,568
Payments on note payable		(13,156)
Proceeds from the issuance of common stock, net	158,414	51,332
Deferred financing costs		(95)
Proceeds from the issuance of preferred stock, net		27,368
Proceeds from exercise of stock options	359
Net cash provided by financing activities	158,773	86,017
NET INCREASE IN CASH AND CASH EQUIVALENTS	15,852	36,068
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	28,518	12,303
CASH AND CASH EQUIVALENTS AT END OF YEAR	44,370	48,371
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	$ 1,253	555
NON-CASH FINANCING ACTIVITIES:
Accretion of dividends on preferred stock		9,000
Fair value of series A-6 convertible preferred stock issued as settlement of liability		10,109
Fair value of warrants issued		1,511
Initial public offering costs included in accrued expenses and other current liabilities		1,182
Reclassification of preferred stock to common stock		$ 298,063