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Warrants (Tables)
 12 Months Ended
Dec. 31, 2013
Warrants and Rights Note Disclosure [Abstract]  
Schedule of Warrant Activity
The following is a summary of warrant activity for the years ended December 31, 2013 and 2012 (in thousands):
 
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
 
Consultant
and Oxford Warrants
 
Total
Outstanding, January 1, 2012
—

 
—

 
9,470

 
2,400

 
540

 
978

 
733

 
14,121

Granted
—

 
7,578

 
—

 
—

 
—

 
—

 
400

 
7,978

Exercised
—

 
—

 
(6,624
)
 
(2,039
)
 
(180
)
 
—

 
(40
)
 
(8,883
)
Outstanding, December 31, 2012
—

 
7,578

 
2,846

 
361

 
360

 
978

 
1,093

 
13,216

Granted
7,044

 
—

 
—

 
—

 
—

 
—

 
182

 
7,226

Exercised
(602
)
 
(2,661
)
 
(1,688
)
 
(185
)
 
(70
)
 
—

 
(196
)
 
(5,402
)
Expired
—

 
—

 
—

 
—

 
—

 
—

 
(190
)
 
(190
)
Outstanding, December 31, 2013
6,442

 
4,917

 
1,158

 
176

 
290

 
978

 
889

 
14,850

Expiration
September 2018
 
December 2017
 
April 2017
 
March 2016
 
March 2016
 
August 2014
 
Varies 2014-2020
 
 
Fair Value of Warrants is Estimated Using Black-Scholes Option Pricing Model
The fair value of the warrants is estimated using an appropriate pricing model with the following inputs:
 
 
As of December 31, 2013
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
Strike price
$
2.50

 
$
1.90

 
$
0.65

 
$
0.65

 
$
2.15

 
$
4.50

Expected term (years)
4.72

 
3.98

 
3.31

 
2.18

 
2.24

 
0.59

Volatility %
71.97
%
 
71.38
%
 
71.71
%
 
73.45
%
 
73.36
%
 
66.85
%
Risk-free rate %
1.61
%
 
1.25
%
 
0.93
%
 
0.45
%
 
0.47
%
 
0.11
%
 
 
As of December 31, 2012
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
Strike price
—

 
$
1.90

 
$
0.65

 
$
0.65

 
$
2.18

 
$
4.50

Expected term (years)
0.00

 
4.98

 
4.30

 
3.18

 
3.24

 
1.59

Volatility %
—

 
80.93
%
 
82.48
%
 
69.90
%
 
69.79
%
 
74.13
%
Risk-free rate %
—

 
0.72
%
 
0.59
%
 
0.39
%
 
0.40
%
 
0.21
%
Changes in Fair Value of Warrant Liability
The changes in fair value of the warrant liability for the years ended December 31, 2013 and 2012 were as follows (in thousands):
 
 
September
2013
Warrants
 
December
2012
Warrants
 
April 2011
Warrants
 
March
2011
Warrants
 
March
2010
Warrants
 
August
2009
Warrants
 
Total
Warrant liability, January 1, 2012
$
—

 
$
—

 
$
3,145

 
$
421

 
$
116

 
$
64

 
$
3,746

Fair value of warrants granted
—

 
7,286

 
—

 
—

 
—

 
—

 
7,286

Fair value of warrants exercised
—

 
—

 
(8,130
)
 
(2,456
)
 
(257
)
 
—

 
(10,843
)
Change in fair value of warrants
—

 
(332
)
 
8,295

 
2,413

 
328

 
71

 
10,775

Warrant liability, December 31, 2012
—

 
6,954

 
3,310

 
378

 
187

 
135

 
10,964

Fair value of warrants granted
8,238

 
—

 
—

 
—

 
—

 
—

 
8,238

Fair value of warrants exercised
(1,931
)
 
(8,482
)
 
(3,455
)
 
(260
)
 
(121
)
 
—

 
(14,249
)
Change in fair value of warrants
16,643

 
19,588

 
5,214

 
645

 
879

 
1,043

 
44,012

Warrant liability, December 31, 2013
$
22,950

 
$
18,060

 
$
5,069

 
$
763

 
$
945

 
$
1,178

 
$
48,965