STATEMENTS OF OPERATIONS (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
STATEMENTS OF OPERATIONS [Abstract]
Government contracts	$ 20,671	$ 14,688	$ 343
Research and development collaborations	1,405
Total revenue	22,076	14,688	343
Costs and expenses:
Cost of government contracts revenue	14,692	7,003
Research and development	26,061	17,885	28,032
General and administrative	10,988	11,379	10,805
Total costs and expenses	51,741	36,267	38,837
Loss from operations before other income (expense)	(29,665)	(21,579)	(38,494)
Other income (expense):
Interest income	165	136	189
Interest expense	(32)	(9)	(9)
Other income	45	26	485
Realized gains on short-term investments	879
Change in fair value of warrant liability	101	2,474	1,671
Loss from operations before income tax	(28,507)	(18,952)	(36,158)
Income tax expense (benefit)		412	450
Net loss	$ (28,507)	$ (19,364)	$ (35,708)
Basic and diluted net loss per share	$ (0.22)	$ (0.17)	$ (0.34)
Basic and diluted weighted average number of common shares outstanding	131,726	113,610	104,768