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Stock-Based Compensation
 6 Months Ended
Jun. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-Based Compensation
As of June 30, 2023, there were 7,047,240 shares subject to outstanding options and restricted stock units (RSUs) under the 2006 Equity Incentive Plan (2006 Plan) and the Amended and Restated 2016 Equity Incentive Plan (2016 Plan, and together with the 2006 Plan, Plans). The 2006 Plan expired by its terms in April 2016, and the Company adopted the 2016 Plan. Outstanding options under the 2006 Plan remain in effect and the terms of the 2006 Plan continue to apply, but no additional awards can be granted under the 2006 Plan. In June 2016, the Company’s stockholders approved the 2016 Plan. The 2016 Plan has been amended a number of times since to increase the number of shares reserved for issuance, among other administrative changes. Each of the amendments to the 2016 Plan was approved by the Company’s stockholders. There is a total of 13,790,000 shares of common stock authorized for issuance under the 2016 Plan, 4,479,515 shares of which remained available for future grant as of June 30, 2023.
Stock Options
The Company has granted option awards under the Plans with service conditions (service option awards) that are subject to terms and conditions established by the compensation committee of the board of directors. Service option awards have 10-year contractual terms. Service option awards granted to employees and new directors upon their election vest and become exercisable over four years, with the first 25% of the shares subject to service option awards vesting on the first anniversary of the grant date and the remaining 75% of the shares subject to the service option awards in 36 equal monthly installments thereafter. Subsequent annual service option awards granted to directors vest and become exercisable in full on the first anniversary of the grant date. Service option awards granted to executive officers and certain other employees provide for partial acceleration of vesting if the executive officer or employee is subject to an involuntary termination, and full acceleration of vesting if the executive officer or employee is subject to an involuntary termination within 24 months after a change in control of the Company. Service option awards granted to directors provide for accelerated vesting if there is a change in control of the Company or if the director’s service terminates as a result of the director’s death or total and permanent disability.
As of June 30, 2023, $8.1 million of unrecognized compensation costs related to unvested service option awards are expected to be recognized over a weighted average period of 1.3 years. No option awards are classified as a liability as of June 30, 2023.
A summary of option activity under the Plans for the six months ended June 30, 2023 follows:
(in thousands, except for share and per share amounts)Number of
Shares		Weighted Average
Exercise Price at
Grant Date		Weighted Average
Remaining Term
(Years)		Aggregate
Intrinsic
Value
Outstanding at December 31, 20224,232,210 $14.19 5.81$— 
Granted944,776 6.92 
Expired(130,417)12.38 
Outstanding at June 30, 20235,046,569 12.88 6.2026 
Exercisable at June 30, 20233,321,471 14.26 4.76— 
Vested and expected to vest at June 30, 20234,763,321 13.15 6.0120 
The weighted average grant date fair value of options granted was $3.53 and $5.18 per share for the six months ended June 30, 2023 and 2022, respectively. There were no proceeds from the exercise of stock options for the six months ended June 30, 2023. Proceeds from the exercise of stock options amounted to $0.1 million for the six months ended June 30, 2022.
Restricted Stock Units
An RSU is a stock award that entitles the holder to receive shares of the Company’s common stock as the award vests. The fair value of each RSU is based on the closing price of the Company’s stock on the date of grant. The Company has granted RSUs under the Plans with service conditions (service RSUs) that are subject to terms and conditions established by the compensation committee of the board of directors. Service RSUs granted to employees and new directors upon their election vest in four equal annual installments. Subsequent annual service RSUs granted to directors vest on the first anniversary of the date of grant. Service RSUs granted to executive officers and certain other employees provide for accelerated vesting if the executive officer or employee is subject to an involuntary termination within 24 months after a change in control. Service RSUs granted to directors provide for accelerated vesting if there is a change in control of the Company.
As of June 30, 2023, $19.0 million of unrecognized compensation costs related to unvested service RSUs are expected to be recognized over a weighted average period of 1.7 years. No RSUs are classified as a liability as of June 30, 2023.
A summary of RSU activity for the Plans for the six months ended June 30, 2023 is as follows:
Number of
Shares
Weighted
Average
Grant Date Fair Value
Unvested at December 31, 20221,915,546 $14.41 
Granted852,243 6.97 
Forfeited(53,969)13.13 
Vested(713,149)15.10 
Unvested at June 30, 20232,000,671 11.03 
The grant date fair value for the 713,149 shares underlying RSUs that vested during the six months ended June 30, 2023 was $10.8 million.
Stock-Based Compensation Expense
Stock-based compensation expense recognized for the three and six months ended June 30, 2023 and 2022 was comprised of the following:
 Three Months EndedSix Months Ended
(in thousands)June 30,
2023		June 30,
2022		June 30,
2023		June 30,
2022
Research and development$733 $899 $1,799 $2,059 
Selling, general and administrative2,516 2,931 5,801 6,549 
Total stock-based compensation expense$3,249 $3,830 $7,600 $8,608 
The fair value of each option award is estimated on the date of grant using the Black-Scholes-Merton option pricing model that uses the assumptions noted in the following table. Expected volatility rates are based on the historical volatility of the Company’s publicly traded common stock and other factors. The expected terms are determined based on a combination of historical exercise data and hypothetical exercise data for unexercised stock options. The risk-free interest rates are based on the
U.S. Treasury yield for a period consistent with the expected term of the option in effect at the time of the grant. The Company has never paid cash dividends to its stockholders and does not plan to pay dividends in the foreseeable future. Assumptions used in the Black-Scholes-Merton option pricing model for employee and director stock options granted during the six months ended June 30, 2023 and 2022 were as follows:
 Six Months Ended
June 30, 2023June 30, 2022
Expected dividend yield0 %0 %
Weighted average expected volatility47 %46 %
Weighted average expected term (years)6.166.05
Weighted average risk-free rate3.89 %2.03 %