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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2021
Stock-Based Compensation  
Schedule of summary of option activity
The following is a summary of stock options and SARs activity under the 2015 Plan and 2005 Plan for the three months ended March 31, 2021:
2015 Plan2005 Plan
Stock
Options and SARs		Weighted-Average
Exercise
Price		Stock
Options		Weighted-Average
Exercise
Price
Outstanding at January 1, 20215,420,463 $38.05 214,186 $88.11 
Granted32,958 $132.14 — $— 
Exercised(437,054)$51.42 (35,401)$106.49 
Canceled(6,270)$27.65 — $— 
Outstanding at March 31, 20215,010,097 $37.52 178,785 $84.47 
Shares exercisable at March 31, 2021590,006 $72.46 178,785 $84.47 
Shares available for grant at March 31, 20212,381,759 
Schedule of stock unit activity
The following is a summary of restricted stock units activity for the three months ended March 31, 2021:
Number of
Shares		Per Share
Weighted-
Average
Fair Value
Outstanding and Unvested at January 1, 20201,044,980 $72.59 
Restricted stock units granted69,732 $181.24 
Restricted stock units vested(26,097)$68.44 
Restricted stock units forfeited(4,263)$142.88 
Outstanding and Unvested at March 31, 20211,084,352 $79.39 
Schedule of stock-based compensation expense
The Company recorded all stock-based compensation expense in the consolidated statements of operations as follows (in thousands):
Three Months Ended
March 31,
20212020
Research and development$23,790 $1,908 
General and administrative29,270 2,057 
Total stock-based compensation expense$53,060 $3,965 
Stock options  
Stock-Based Compensation  
Schedule of assumptions used to estimate grant date fair value of stock options and stock appreciation rights granted using black-scholes option-pricing model The fair value of stock options granted under the 2015 Plan was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:
Three Months Ended
March 31,
20212020
Weighted average Black-Scholes fair value of stock options granted$116.26$5.39
Risk-free interest rate
0.5%-0.9%
0.6%-1.5%
Dividend yield—%—%
Volatility
124.7%-140.3%
133.6%-142.6%
Expected term (in years)
4.1-5.3
3.9
ESPP  
Stock-Based Compensation  
Schedule of assumptions used to estimate grant date fair value of stock options and stock appreciation rights granted using black-scholes option-pricing model
The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:
Three Months Ended
March 31,
20212020
Range of Black-Scholes fair values of ESPP shares granted
$128.70-$238.85
$2.57-$35.00
Risk-free interest rate0.1%
1.5%-2.6%
Dividend yield—%—%
Volatility
120.4%-159.4%
66.6%-154.4%
Expected term (in years)
0.5-2.0
0.5-2.0