Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (49,784)	$ (36,375)	$ (26,088)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	236	160	71
Amortization of debt discount and beneficial conversion	29	15	459
Amortization of debt issuance costs	32	16	19
Amortization of premiums and discounts on investments	647	202	47
Revaluation of warrants			2,587
Noncash interest expense on convertible notes			459
Stock-based compensation expense	12,726	3,679	1,227
Loss related to assets held for sale		29	27
(Gain)/Loss on sale of assets	47	2	(148)
Changes in assets and liabilities:
Prepaids and other assets	(1,275)	(264)	27
Accounts payable	(1,333)	(299)	1,756
Other accrued liabilities	519	814	1,443
Net cash used in operating activities	(38,156)	(32,021)	(18,114)
Investing activities
Purchases of investments	(280,559)	(48,088)	(24,677)
Proceeds from sales/maturities of investments	120,792	12,351	3,505
Proceeds from sale of assets	24	12	201
Purchase of property and equipment	(325)	(873)	(31)
Net cash used in investing activities	(160,068)	(36,598)	(21,002)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	189,983	91,731	72,194
Proceeds from issuance of preferred stock, net of issuance costs			16,824
Proceeds from exercise of common stock options	1,177	473	123
Proceeds from warrant issuance		78
Proceeds from debt issuance, net of issuance costs		4,838
Payments of long term debt	(638)
Net cash provided by financing activities	190,522	97,120	89,141
Net (decrease) increase in cash and cash equivalents	(7,702)	28,501	50,025
Cash and cash equivalents at beginning of period	85,038	56,537	6,512
Cash and cash equivalents at end of period	$ 77,336	$ 85,038	56,537
10% Convertible promissory notes
Supplemental disclosure of cash flow information:
Conversion of promissory notes, including accrued interest into Series A - 1 preferred stock			16,623
Pfizer note
Supplemental disclosure of cash flow information:
Conversion of promissory notes, including accrued interest into Series A - 1 preferred stock			$ 7,803