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Stock Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2021
Stock Based Compensation [Abstract]  
Weighted-Average Assumptions for Stock Options

The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the stock options granted to employees, non-employees and directors were as follows:

  
Three Months Ended March 31,
  
 
  
2021
    
2020
  
 
      
Risk-free interest rate
    
0.62
%
    
1.48
%
Expected term (in years)
    
5.87
      
5.87
  
Expected volatility
    
69.41
%
    
77.75
%
Expected dividend yield
    
0.00
%
    
0.00
%
Exercise price
  
$
36.99
    
$
22.47
  
Fair value of common stock
  
$
36.99
    
$
22.47
  
Stock Option Activity

The following table summarizes stock option activity for the three months ended March 31, 2021 under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

 
  
Number of
Shares
    
Weighted
Average
Exercise
Price
    
Weighted
Average
Contractual
Term (Years)
    
Aggregate
Intrinsic
Value
  
 
                        
Outstanding as of December 31, 2020
    
11,050,931
    
$
9.10
      
6.55
    
$
504,079
  
Granted
    
916,575
      
60.95
      
10.64
          
Exercised
    
(991,432
)
    
8.31
              
51,892
  
Cancelled
    
(83,244
)
    
31.82
                  
Outstanding as of March 31, 2021
    
10,892,830
    
$
13.42
      
6.57
    
$
349,406
  
 
                                
Options vested and exercisable as of March 31, 2021
    
7,856,137
    
$
5.70
      
5.55
    
$
300,452
  
Options unvested as of March 31, 2021
    
3,036,693
    
$
33.17
      
9.20
          
Stock-Based Compensation Expense by Award Type

Stock-based compensation expense recognized by award type was as follows:

  
Three Months Ended March 31,
  
 
  
2021
    
2020
  
 
      
Stock options
  
$
7,826
    
$
3,961
  
Restricted stock units
    
74
      
-
  
Total share based compensation expense
  
$
7,900
    
$
3,961
  
Stock-Based Compensation Expense

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows:

  
Three Months Ended March 31,
  
 
  
2021
    
2020
  
 
      
Research and development
  
$
2,916
    
$
1,735
  
General and administrative
    
4,984
      
2,226
  
Total share based compensation expense
  
$
7,900
    
$
3,961
  
RSU Activity

The following table summarizes our RSU activity for the three months ended March 31, 2021:

  
Number of
Shares
    
Weighted
Average
Grant Date
Fair Value
  
             
Unvested as of December 31, 2020
    
20,000
    
$
25.06
  
Granted
    
3,500
      
62.32
  
Vested
    
-
      
-
  
Forfeited
    
-
      
-
  
Unvested as of March 31, 2021
    
23,500
      
30.61