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Equity Incentive Plans
 6 Months Ended
Jun. 30, 2021
Share-based Payment Arrangement [Abstract]  
Equity Incentive Plans EQUITY INCENTIVE PLANS
Stock-Based Compensation
Stock-based compensation expense recognized for stock options, restricted stock units ("RSUs"), performance-based restricted stock units ("PRSUs") and our employee stock purchase program (the "ESPP") are recorded as operating expenses in our condensed statements of operations and comprehensive loss, as follows (in thousands):
Three Months Ended June 30,Six Months Ended June 30,
2021202020212020
Research and development$1,087 $1,144 $2,179 $2,202 
General and administrative2,132 1,530 4,127 3,420 
Total$3,219 $2,674 $6,306 $5,622 
As of June 30, 2021, our total unrecognized stock-based compensation expense, net of estimated forfeitures, and average remaining vesting period, included the following (dollars in thousands):
Unrecognized Compensation
 Expense		Average Remaining
 Vesting Period (Years)
Stock options$25,267 2.8
RSUs$6,978 3.6
ESPP$151 0.2
Stock Options
A summary of our stock option activity and related information for the six months ended June 30, 2021 is as follows (in thousands, except dollar amounts):
Number of SharesWeighted Average Exercise Price
Outstanding balance at December 31, 20209,790 $6.76 
Granted3,328 $6.62 
Exercised(205)$2.75 
Forfeited(401)$7.56 
Outstanding balance at June 30, 202112,511 $6.76 
Exercisable at June 30, 20216,043 $7.38 
Restricted Stock Units
A summary of our RSUs activity and related information for the six months ended June 30, 2021 is as follows (in thousands, except dollar amounts):
Number of RSUsWeighted Average Value at Grant
Outstanding balance at December 31, 2020159 $5.64 
Granted1,088 $6.62 
Vested(80)$6.43 
Forfeited(32)$6.10 
Unvested at June 30, 20211,135 $6.52 
We issued no PRSUs during the six months ended June 30, 2021 and we have no PRSUs outstanding as of June 30, 2021.
Employee Stock Purchase Plan
In February 2021, we sold approximately 0.2 million shares of our common stock under the ESPP. The shares were purchased by employees at a purchase price of $5.84 per share resulting in proceeds to us of approximately $1.4 million.
Issuance of Common Stock for Services
Under Our Amended and Restated Non-Employee Director Compensation Program, members of our board of directors may elect to receive shares of our stock in lieu of their cash fees. During the six months ended June 30, 2021, we issued no shares of our common stock to members of the board of directors in accordance with the program.