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Stockholders' Equity (Tables)
 3 Months Ended
Mar. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Common Stock Reserved for Future Issuance
The following shares of common stock were reserved for future issuance as of March 31, 2016:
 
Common stock options outstanding
6,624,083

Common stock available for future grant under 2012 Equity Incentive Plan
2,491,184

Common stock available for future grant under 2015 Inducement Plan
581,806

Employee Stock Purchase Plan
1,697,431

Total common shares reserved for future issuance
11,394,504
Stock Option Activity
The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2016 (shares in thousands): 
 
Number of
options
 
Weighted
average
exercise
price
Options outstanding at December 31, 2015
5,126

 
$
8.91

Granted
1,960

 
$
6.75

Exercised
(43
)
 
$
4.83

Canceled/forfeited/expired
(419
)
 
$
10.61

Options outstanding at March 31, 2016
6,624

 
$
8.19
Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan
The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2012 Equity Incentive Plan and 2015 Inducement Plan and the shares purchasable under our Employee Stock Purchase Plan during the periods presented:
 
 
Three months ended
March 31,
 
2016
 
2015
Stock options
 
 
 
Risk-free interest rate
1.4
%
 
1.7
%
Volatility
79.9
%
 
75.1
%
Dividend yield
—

 
—

Expected term (years)
5.9

 
6.1

Performance stock options
 
Risk-free interest rate
1.5
%
 
1.8
%
Volatility
79.4
%
 
76.4
%
Dividend yield
—

 
—

Expected term (years)
6.0

 
6.0

Employee stock purchase plan shares
 
Risk-free interest rate
0.3
%
 
0.1
%
Volatility
79.2
%
 
72.9
%
Dividend yield
—

 
—

Expected term (years)
0.5

 
0.5
Stock-Based Compensation
The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands): 
 
Three months ended
March 31,
 
2016
 
2015
Research and development
$
1,432

 
$
1,956

General and administrative
2,317

 
1,147

Total
$
3,749

 
$
3,103