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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2013
Stock-Based Compensation Expense Related to Common Stock Options, Common Stock, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, common stock, restricted common stock and common stock warrants:

 

     Three Months Ended
March  31,
 
           2013                  2012        

Research and development

      $ 319           $ 149    

General and administrative

     533          324    
  

 

 

    

 

 

 

Total stock-based compensation expense

      $       852           $       473    
  

 

 

    

 

 

 
Summary of Stock Option Activity

The following table summarizes the stock option activity in the Company’s equity incentive plans, including non-plan grants to Company executives, from December 31, 2012 through March 31, 2013:

    

 

          Shares          

       Weighted Average  
Exercise Price
 

Outstanding, December 31, 2012

     3,539,961        $                     5.66       

Granted

     200,426         3.36       

Exercised

     (39,762)         2.77       

Options forfeited/cancelled

     (44,408)         9.50       
  

 

 

    

Outstanding, March 31, 2013

     3,656,217        $ 5.52       
  

 

 

    
Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of all other options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used (there were no option grants to employees or consultants during the three months ended March 31, 2013):

 

     Three
    Months Ended     
March 31,
      

Cumulative

Period from

Inception

 (July 10, 2000) to   
March 31,

     2013  

 

   2013

Risk-free interest rate

   0.80%      1.76%

Expected life of the options

   5.0 years      5.2 years

Expected volatility of the underlying stock

   117%      119%

Expected dividend rate

   0%      0%