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Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock-Based Compensation Expense

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

 

     Three Months Ended September 30,      Nine Months Ended September 30,  
     2014      2013      2014      2013  

Research and development

   $ 380       $ 113       $ 1,062       $ 285   

General and administrative

     476         24         1,187         109   
  

 

 

    

 

 

    

 

 

    

 

 

 

Total stock-based compensation expense

   $ 856       $ 137       $ 2,249       $ 394   
  

 

 

    

 

 

    

 

 

    

 

 

 
Schedule of Fair Value Option Award Estimated on the Date of Grant Using the Black-Scholes Option-Pricing Model

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,
     2014   2013

Expected dividend yield

   0%   0%

Expected volatility

   67%   53%-58%

Risk-free interest rate

   2.04%-2.32%   1.24%-2.05%

Expected term

   6.25 years   7 years

Expected forfeiture rate

   5%   5.06%
Schedule of Stock Option Activity

The following table summarizes stock option activity under the Plan during the nine months ended September 30, 2014:

 

     Shares     Weighted-
Average
Exercise Price
     Weighted-Average
Remaining
Contractual Term
(Years)
     Aggregate
Intrinsic
Value
(in thousands)
 

Outstanding, December 31, 2013

     3,200,958      $ 4.90         6.9      

Granted

     279,894        20.72         

Exercised

     (307,204     1.41         

Forfeited or expired

     (44,698     7.42         
  

 

 

         

Outstanding, September 30, 2014

     3,128,950        6.62         6.7       $ 46,144   
  

 

 

         

September 30, 2014:

          

Exercisable

     1,756,293        2.45         5.0         32,646   

Vested and expected to vest

     2,955,117        6.35         6.5         44,305