STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Activities:
Net loss	$ (51,983)	$ (28,507)	$ (19,364)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	(267)	(101)	(2,474)
Depreciation and amortization	2,591	1,666	1,613
Loss on disposal of property and equipment	(32)	(28)
Impairment of long-lived assets			360
Amortization of net premiums on investments	507	(18)	317
Deferred rent	897	660	(341)
Non-cash stock-based compensation	2,480	2,091	2,047
Realized gains on investments		(879)
Other non-cash income	(200)
Changes in operating assets and liabilities:
Restricted cash	(431)	(986)
Accounts receivables	(451)	954	(1,911)
Unbilled receivables	(3,418)	266	(1,836)
Prepaid expenses and other assets	402	40	(1,854)
Accounts payable and accrued expenses	4,184	2,009	(2,686)
Deferred revenue	(341)	258	2,500
Lease incentives received	703	4,346
Net cash used in operating activities	(45,359)	(18,229)	(23,629)
Investing Activities:
Capital expenditures	(5,785)	(4,341)	(610)
Proceeds from disposal of property and equipment	116	324
Net cash received from the Isconova AB acquisition	3,034
Purchases of investments	(14,754)	(48,652)	(2,082)
Proceeds from maturities of investments	33,781	20,407	21,235
Net cash provided by (used in) investing activities	16,392	(32,262)	18,543
Financing Activities:
Principal payments of capital leases	(87)	(104)
Principal payments of notes payable	(473)	(60)	(80)
Proceeds from notes payable	1,450	650
Proceeds from settlement of notes receivable from former directors			50
Restricted cash	(1)	(756)
Net proceeds from sales of common stock, net of offering costs of $6.1 million, $0.4 million and $0.2 million, respectively	128,648	54,002	10,980
Proceeds from the exercise of stock options	1,498	54	179
Net cash provided by financing activities	131,035	53,786	11,129
Effect of exchange rate on cash and cash equivalents	4
Net increase in cash and cash equivalents	102,072	3,295	6,043
Cash and cash equivalents at beginning of period	17,399	14,104	8,061
Cash and cash equivalents at end of period	119,471	17,399	14,104
Supplemental disclosure of non-cash activities:
Common stock issued in connection with the Isconova AB acquisition	41,942
Capital expenditures included in accounts payable and accrued expenses	379	1,321	14
Deposit applied towards the purchase of laboratory equipment		500
Equipment acquired under a capital lease		399
Settlement of notes receivable from former directors			1,522
Sale of common stock under the 2010 Sales Agreement not settled at year-end			847
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 177	$ 20