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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2020
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 March 31,
20202019
Research and development$2,434  $1,814  
General and administrative2,057  1,985  
Total stock-based compensation expense$4,491  $3,799  
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:
Three Months Ended March 31,
20202019
Expected dividend yield0%0%
Expected volatility67.3% - 69.5%73.7% - 74.4%
Risk-free interest rate0.6% - 1.8%2.3% - 2.6%
Expected term6.25 years6.25 years
Stock Option and Restricted Stock Unit Activity
The following table summarizes stock option activity during the three months ended March 31, 2020:
SharesWeighted-
Average
Exercise Price
Weighted-Average
Remaining
Contractual Term
(Years)
Aggregate
Intrinsic
Value
(in thousands)
Outstanding, December 31, 20196,706,994  $22.33  6.9
Granted1,403,627  11.49  
Exercised(172,387) 0.94  
Forfeited or expired(293,295) 18.97  
Outstanding, March 31, 20207,644,939  20.95  7.1$1,185  
As of March 31, 2020:
Exercisable4,358,640  23.05  5.61,184  
Vested and expected to vest7,306,954  21.04  7.01,185  
Schedule of RSU Activity
The following table summarizes RSU activity during the three months ended March 31, 2020:

Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2019452,500  $15.32  
Granted—  —  
Exercised—  —  
Forfeited or expired(19,400) 15.32  
Outstanding, March 31, 2020433,100  15.32