Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2015
Income Tax Disclosure [Abstract]  
Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision

The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for (benefit from) income taxes is as follows:

 

     YEARS ENDED DECEMBER 31,  
         2015             2014             2013      

Federal statutory rate

      34.0 %      34.0 %      34.0 % 

Effect of:

       

Impact of foreign rate differential

      (12.6 )      —          —     

Change in valuation allowance

      (24.4 )      (32.0 )      (35.4 ) 

Research and development tax credit

      0.9        0.8        2.5   

Stock-based compensation

      (0.9 )      (2.6 )      (1.3 ) 

Other

      3.0        (0.2 )      0.2   
  

 

 

    

 

 

    

 

 

  

Total

            0.0 %            0.0 %            0.0 % 
  

 

 

    

 

 

    

 

 

  

Deferred Tax Assets

The components of the deferred tax assets are as follows:

 

     As of December 31,  
     2015      2014  

Deferred tax assets:

      

Net operating loss carryforwards

    $ 37,017       $ 41,846   

Capitalized research and development costs

      2,431         2,646   

Research and development credit carryforwards

      3,510         2,360   

Stock compensation

      5,369         2,479   

Depreciation and other costs

      (74 )       372   
  

 

 

     

 

 

  

Net deferred tax assets

      48,253         49,703   

Valuation allowance

      (48,253 )       (49,703 ) 
  

 

 

     

 

 

  

Net deferred tax assets

    $ —        $ —    
  

 

 

     

 

 

  

Unrecognized Tax Benefit

A reconciliation of the gross unrecognized tax benefit is as follows (in thousands):

 

     Year Ended 
December 31,		  
         2015              2014      

Unrecognized tax benefits at the beginning of the period

    $ 1,216       $    865   

Additions for current tax positions

      421         220   

Changes for previous tax positions

      (207 )       131   
  

 

 

     

 

 

  

Unrecognized tax benefits at the end of the period

    $ 1,430       $ 1,216