Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2015
Income Tax Disclosure [Abstract]  
Reconciliation of U.S. Federal Statutory Income Tax Rate

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:

 

     Year Ended December 31,  
     2015     2014     2013  

Federal statutory income tax rate

      (34.0 )%      (34.0 )%      (34.0 )% 

Federal research and development tax credit

      (0.5 )      (0.9 )      (0.7 ) 

State taxes, net of federal benefit

      (5.3 )      (4.5 )      (5.6 ) 

Stock-based compensation expense

      0.4        1.1        0.4   

Other

      0.1        0.2        0.1   

Increase in deferred tax asset valuation allowance

      39.3        38.1        39.8   
  

 

 

    

 

 

    

 

 

  

Effective income tax rate

      0.0 %      0.0 %      0.0 % 
  

 

 

    

 

 

    

 

 

  

Schedule of Net Deferred Tax Assets

Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following:

 

     December 31,  
     2015      2014  

Deferred tax assets:

      

Capitalized research and development expenses

    $ 37,765       $ 24,945   

Net operating loss carryforwards

      10,224         6,292   

Stock-based compensation

      1,371         612   

Research and development tax credit carryforwards

      1,273         1,008   

Accrued expenses

      339         414   

Other

      56         1   
  

 

 

     

 

 

  

Total gross deferred tax assets

      51,028         33,272   

Valuation allowance

      (51,028 )       (33,272 ) 
  

 

 

     

 

 

  

Net deferred tax assets

    $ —         $ —     
  

 

 

     

 

 

  

Summary of Changes in Valuation Allowance for Deferred Tax Assets

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2015, 2014 and 2013 related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and development tax credit carryforwards and were as follows:

 

     Year Ended December 31,  
     2015      2014      2013  

Valuation allowance as of beginning of year

    $ 33,272       $ 24,402       $ 18,138   

Decreases recorded as benefit to income tax provision

      —           —           —     

Increases recorded to income tax provision

      17,756         8,870         6,264   
  

 

 

     

 

 

     

 

 

  

Valuation allowance as of end of year

    $ 51,028       $ 33,272       $ 24,402