Stock-Based Awards (Tables)
 12 Months Ended
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Fair Value of Stock Options Granted

The following table sets forth the assumptions that the Company used to determine the fair value of the stock options granted, presented on a weighted average basis:

 

     Year Ended December 31,  
       2015         2014         2013    

Risk-free interest rate

      1.79 %      1.83 %      1.72 % 

Expected term (in years)

      6.01        5.95        5.98   

Expected volatility

      74.2 %      79.4 %      79.7 % 

Expected dividend yield

      0 %      0 %      0 % 

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity from January 1, 2015 through December 31, 2015:

 

     Shares Issuable
Under Options		      Weighted
Average
Exercise
Price		      Weighted
Average
Remaining
Contractual
Term		      Aggregate
Intrinsic
Value		  
                   (In years)         

Outstanding as of December 31, 2014

      2,146,927       $ 5.54         8.6       $ 19,802   

Granted

      996,845         10.60         

Exercised

      (201,153 )       2.31         

Forfeited

      (81,608 )       4.27         
  

 

 

           

Outstanding as of December 31, 2015

      2,861,011       $ 7.57         8.3       $ 7,906   
  

 

 

           

Options vested and expected to vest as of December 31, 2015

      2,822,786       $ 7.52         8.2       $ 7,896   

Options exercisable as of December 31, 2015

      1,199,676       $ 4.10         7.0       $ 6,455   

Summary of Restricted Stock Unit Activity

The table below summarizes the Company’s restricted stock unit activity from January 1, 2015 through December 31, 2015:

 

     Shares      Weighted
Average Grant
Date Fair Value		  

Unvested restricted common stock units as of December 31, 2014

      54,604       $ 15.00   

Issued

      —           —     

Vested

      (13,651 )       15.00   

Forfeited

      —           —     
  

 

 

     

Unvested restricted common stock units as of December 31, 2015

      40,953       $ 15.00   
  

 

 

     

Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock Units

The Company recorded stock-based compensation expense related to stock options and restricted common stock units in the following expense categories of its statements of operations:

 

     Year Ended December 31,  
     2015      2014      2013  

Research and development

    $ 634       $ 552       $ 91   

General and administrative

      2,267         1,556         147   
  

 

 

     

 

 

     

 

 

  
   $ 2,901       $ 2,108       $ 238