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Stock-Based Compensation
 9 Months Ended
Sep. 30, 2016
Stock-Based Compensation [Abstract]  
Stock-Based Compensation
6. Stock-Based Compensation

 Under the provisions of the Company's 2013 Stock Incentive Plan (2013 Plan), the number of shares of common stock reserved for issuance will automatically increase on January 1 of each year from January 1, 2014 through and including January 1, 2023, by the lesser of (a) 1,960,168 shares, (b) 4.0 % of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or (c) the number of shares of common stock determined by the Board of Directors. During the nine months ended September 30, 2016, the maximum number of shares of common stock authorized to be issued by the Company under the 2013 Plan was increased to 5,375,064.   As of September 30, 2016, there were options to purchase an aggregate of 2,621,944 shares of common stock outstanding at a weighted average exercise price of $26.61 per share under the 2013 Plan.


The following stock-based compensation amounts were recognized for the periods indicated (in thousands):

 
Three Months Ended September 30,
 
Nine Months Ended September 30,
 
 
2016
 
2015
 
2016
 
2015
 
Research and development
 
$
1,403
  
$
916
  
$
4,243
  
$
2,607
 
General and administrative
  
1,600
   
1,182
   
4,883
   
3,024
 
Total stock-based compensation expense
 
$
3,003
  
$
2,098
  
$
9,126
  
$
5,631
 

Employee Stock Options

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:

 
Nine Months Ended September 30,
 
2016
 
2015
Expected dividend yield
0%
 
0%
Expected volatility
 63.7 % - 67.7 %
 
74 % - 75 %
Risk-free interest rate
1.2% - 2.1%
 
1.6% - 2.1%
Expected term
6.25 years
 
6.25 years

The following table summarizes stock option and restricted stock unit (RSU) activity during the nine months ended September 30, 2016:

  
Shares
  
Weighted-
Average
 Exercise Price
  
Weighted-Average
Remaining
Contractual Term
 (Years)
 
Aggregate
Intrinsic
Value
 (in thousands)
 
Outstanding, December 31, 2015
  
4,146,064
  
$
16.90
   
7.4
   
Granted
  
346,524
   
22.50
       
Exercised
  
(469,442
)
  
3.07
       
Forfeited or expired
  
(166,494
)
  
26.50
       
Outstanding, September 30, 2016
  
3,856,652
   
18.67
   
7.2
  
$
45,214
 
As of September 30, 2016:
                
Exercisable
  
2,160,650
   
12.86
   
6.1
   
37,400
 
Vested and expected to vest
  
3,654,153
   
18.32
   
7.1
   
44,082
 

The weighted-average grant-date fair value of options granted for the nine months ended September 30, 2016 was $14.81. The total intrinsic value of options exercised during the nine months ended September 30, 2016 was approximately $9.6 million, and the total cash received for options exercised was approximately $1.4 million. The total fair value of shares vested in the nine months ended September 30, 2016 was approximately $8.8 million. As of September 30, 2016, the total unrecognized compensation expense related to non-vested stock options, net of related forfeiture estimates, was approximately $24.2 million, which the Company expects to recognize over a weighted-average period of approximately three years.