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Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Expense The following stock-based compensation expense was recognized for the periods indicated (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Research and development$3,109 $3,059 $8,894 $8,082 
Selling, general and administrative3,249 2,773 8,820 7,337 
Total stock-based compensation expense$6,358 $5,832 $17,714 $15,419 
Valuation Assumptions Using the Black-Scholes Option-Pricing Model The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:
Nine Months Ended September 30,
20212020
Expected dividend yield0%0%
Expected volatility
86.2% -87.4%
67.3% - 109%
Risk-free interest rate
0.6% - 1.4%
0.4% - 1.8%
Expected term6.25 years6.25 years
Schedule of Stock Options Roll Forward The following table summarizes stock option activity during the nine months ended September 30, 2021:
SharesWeighted-
Average
Exercise Price
Weighted-Average
Remaining
Contractual Term
(Years)
Aggregate
Intrinsic
Value
 (in thousands)
Outstanding, December 31, 20207,258,353 $21.48 6.8
Granted1,914,846 20.86 
Exercised(302,649)17.77 
Forfeited or expired(339,961)19.12 
Outstanding, September 30, 20218,530,589 21.52 6.7$19,925 
As of September 30, 2021:
Exercisable5,314,252 22.76 5.510,445 
Vested and expected to vest8,135,407 21.62 6.718,793 
Schedule of RSU Activity The following table summarizes RSU activity during the nine months ended September 30, 2021:
Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2020209,250 $15.92 
Granted16,500 26.27 
Exercised(184,100)15.66 
Forfeited or expired(17,650)15.32 
Outstanding, September 30, 202124,000 25.46