STATEMENTS OF OPERATIONS (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
STATEMENTS OF OPERATIONS [Abstract]
Government contracts	$ 3,276	$ 7,103	$ 6,717	$ 11,745
Research and development collaborations	255		648
Total revenue	3,531	7,103	7,365	11,745
Costs and expenses:
Cost of government contracts revenue	1,632	5,118	3,344	8,903
Research and development	10,785	5,371	20,041	10,627
General and administrative	4,012	2,469	6,882	5,537
Total costs and expenses	16,429	12,958	30,267	25,067
Loss from operations	(12,898)	(5,855)	(22,902)	(13,322)
Other income (expense):
Interest income	48	39	95	72
Interest expense	(45)	(3)	(68)	(6)
Change in fair value of warrant liability	267	(101)	267
Loss from operations before income tax	(12,628)	(5,920)	(22,608)	(13,256)
Income tax expense	5		22
Net loss	$ (12,633)	$ (5,920)	$ (22,630)	$ (13,256)
Basic and diluted net loss per share	$ (0.08)	$ (0.05)	$ (0.15)	$ (0.11)
Basic and diluted weighted average number of common shares outstanding	152,312	126,925	150,391	123,741