Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (101,526)	$ (62,479)	$ (60,690)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	176	77	27
Amortization of premium (accretion of discount) on marketable securities, net	1,714	429	27
Stock-based compensation expense	14,734	7,070	1,508
Research and development expense settled in stock		2,717	13,118
Change in fair value of other current assets, warrant liability and other liability		505	(9,087)
Non-cash interest	183	295	387
Loss on retirement of note payable	1,572	57
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,914)	(1,639)	1,721
Other long-term assets	(108)	(105)
Accounts payable	3,936	1,991	(250)
Accrued expenses and other current liabilities	(2,870)	2,737	8,222
Net cash used in operating activities	(87,103)	(48,345)	(45,017)
CASH FLOWS (USED IN) PROVIDED BY INVESTING ACTIVITIES:
Purchases of property and equipment	(1,231)	(857)	(2)
Purchases of marketable securities	(579,088)	(97,678)	(17,070)
Sales and maturities of marketable securities	340,497	20,470	21,043
Net cash (used in) provided by investing activities	(239,822)	(78,065)	3,971
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options	2,337	170	13
Proceeds from the issuance of preferred stock, net		27,368	42,870
Proceeds from note payable		24,555
Proceeds from the issuance of common stock, net	482,250	103,804
Deferred financing costs		(116)
Payments on note payable	(25,000)	(13,156)	(8,187)
Fee for early prepayment of note payable	(1,502)
Net cash provided by financing activities	458,085	142,625	34,696
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	131,160	16,215	(6,350)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	28,518	12,303	18,653
CASH AND CASH EQUIVALENTS AT END OF PERIOD	159,678	28,518	12,303
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	$ 1,490	1,971	1,796
NON-CASH FINANCING ACTIVITIES:
Accretion of dividends on preferred stock		9,000	17,471	[1]
Reclassification of Preferred Stock to Common Stock		298,063
Fair value of series A-6 convertible preferred stock issued as settlement of liability		10,109	23,168
Fair value of warrants issued		$ 1,552	$ 1,356

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