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Stockholders' Equity (Tables)
 9 Months Ended
Sep. 30, 2017
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 September 30, 2017:
 
Common stock options outstanding
10,483,174

Common stock available for future grant under 2012 Equity Incentive Plan
1,240,667

Common stock available for future grant under 2015 Inducement Plan
41,948

Employee Stock Purchase Plan
1,675,110

Total common shares reserved for future issuance
13,440,899
Stock Option Activity
The following table summarizes our stock option activity under all equity incentive plans for the nine months ended September 30, 2017 (shares in thousands): 
 
Number of
options
 
Weighted
average
exercise
price
Options outstanding at December 31, 2016
8,931

 
$
6.70

Granted
4,523

 
$
1.41

Exercised
(11
)
 
$
0.38

Canceled/forfeited/expired
(2,960
)
 
$
6.03

Options outstanding at September 30, 2017
10,483

 
$
4.62
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
September 30,
 
Nine months ended
September 30,
 
2017
 
2016
 
2017
 
2016
Stock options
 
 
 
 
 
 
 
Risk-free interest rate
1.9
%
 
1.5
%
 
2.0
%
 
1.4
%
Volatility
89.5
%
 
80.0
%
 
89.4
%
 
79.9
%
Dividend yield
—

 
—

 
—

 
—

Expected term (years)
6.1

 
5.7

 
6.1

 
5.9

Performance stock options
 
 
 
Risk-free interest rate
—

 
—

 
2.1
%
 
1.4
%
Volatility
—

 
—

 
89.9
%
 
79.3
%
Dividend yield
—

 
—

 
—

 
—

Expected term (years)
0

 
0

 
5.6

 
6.0

Employee stock purchase plan shares
 
 
 
Risk-free interest rate
1.0
%
 
0.5
%
 
0.8
%
 
0.5
%
Volatility
104.5
%
 
94.5
%
 
110.7
%
 
85.4
%
Dividend yield
—

 
—

 
—

 
—

Expected term (years)
0.5

 
0.5

 
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, including the adjustments to stock-based compensation expense associated with our May 2017 corporate restructuring (in thousands): 
 
Three months ended
September 30,
 
Nine months ended
September 30,
 
2017
 
2016
 
2017
 
2016
Research and development
$
653

 
$
1,199

 
$
2,411

 
$
3,931

Research and development-restructuring related adjustments
—


—


(1,399
)

—

General and administrative
631

 
2,268

 
2,840

 
5,522

General and administrative-restructuring related adjustments
—

 
—

 
2,679

 
—

Total
$
1,284

 
$
3,467

 
$
6,531

 
$
9,453