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Stock-Based Compensation (Tables)
6 Months Ended
Jun. 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 June 30,Six Months Ended June 30,
2021202020212020
Research and development$3,058 $2,589 $5,785 $5,023 
Selling, general and administrative3,012 2,506 5,571 4,564 
Total stock-based compensation expense$6,070 $5,095 $11,356 $9,587 
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:
Six Months Ended June 30,
20212020
Expected dividend yield0%0%
Expected volatility
86.2% -86.7%
67.3% - 108.8%
Risk-free interest rate
0.6% - 1.4%
0.5% - 1.8%
Expected term6.25 years6.25 years
Stock Option and Restricted Stock Unit Activity The following table summarizes stock option activity during the six months ended June 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
Granted1,790,246 20.72 
Exercised(250,175)18.35 
Forfeited or expired(187,526)16.36 
Outstanding, June 30, 20218,610,898 21.48 7.0$53,928 
As of June 30, 2021:
Exercisable4,957,615 22.96 5.625,634 
Vested and expected to vest8,194,010 21.58 6.950,662 
Schedule of RSU Activity The following table summarizes RSU activity during the six months ended June 30, 2021:
Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2020209,250 $15.92 
Granted10,500 29.17 
Exercised(7,500)23.68 
Forfeited or expired(17,400)15.32 
Outstanding, June 30, 2021194,850 16.39