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Stock-Based Compensation
 6 Months Ended
Jun. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock Based CompensationOur Board of Directors (the Board) and the requisite stockholders previously approved the 2010 Equity Incentive Plan (the 2010 Plan). In October 2013, the Board approved the 2013 Equity Incentive Plan (the 2013 Plan), and in
November 2013, our stockholders approved the 2013 Plan, which became effective as of December 3, 2013. As of December 2, 2013, we suspended the 2010 Plan, and no additional awards may be granted under the 2010 Plan.
Unless otherwise determined by the Board, beginning January 1, 2014, and continuing until the expiration of the 2013 Plan, the total number of shares of common stock available for issuance under the 2013 Plan will automatically increase annually on January 1 of each year by 4% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year. Pursuant to approval by the Board, the total number of shares of common stock available for issuance under the 2013 Plan was increased by 2,399,908 shares on January 1, 2023.
In June 2023, the Board and shareholders approved the 2023 Equity Incentive Plan (the 2023 Plan), which became effective as of June 14, 2023. We suspended the 2013 Plan, and no additional award may be granted under the 2013 Plan. The 2023 Plan reserve consists of 3,000,000 shares and the remaining available shares from the 2013 Plan as of the effective date of the 2023 Plan. In addition, any shares of common stock covered by awards granted under the 2013 Plan that terminate on or after June 14, 2023 by expiration, forfeiture, cancellation, or other means without the issuance of such shares will be added to the 2023 Plan reserve.
As of June 30, 2023, the total number of shares of common stock available for issuance under the 2023 Plan is 19,790,382, which includes 16,932,548 shares of common stock that were available for issuance under the Prior Plans as of the effective date of the 2023 Plan. As of June 30, 2023, a total of 16,476,718 options have been granted under the 2013 Plan and 2023 Plan.
In November 2013, the Board and our stockholders approved the ESPP, which became effective as of December 5, 2013. As of June 30, 2023, the total number of shares of common stock available for issuance under the ESPP is 1,084,060. Unless otherwise determined by the Board, beginning on January 1, 2014, and continuing until the expiration of the ESPP, the total number of shares of common stock available for issuance under the ESPP will automatically increase annually on January 1 by the lesser of (i) 1% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year, or (ii) 621,814 shares of common stock. Pursuant to approval by the Board, the total number of shares of common stock available for issuance under the ESPP was increased by 599,977 shares on January 1, 2023. As of June 30, 2023, we have issued a total of 690,758 shares of common stock under the ESPP.
During the six months ended June 30, 2023, the Company awarded 944,726 RSUs to certain employees. The standard vesting of these awards is generally in three equal annual installments and is contingent on an employee’s continued service to the Company. The fair value of these awards is determined based on the intrinsic value of the stock on the date of grant and will be recognized as stock-based compensation expense over the requisite service period. As of June 30, 2023, a total of 2,944,543 RSUs have been granted under the 2013 Plan and 2023 Plan.
Total employee, director and non-employee stock-based compensation expense recognized for the three and six months ended June 30, 2023 and 2022 are as follows (in thousands):
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
General and administrative $4,471 $4,350 $8,747 $8,024 
Research and development 9,092 8,253 17,415 15,384 
$13,563 $12,603 $26,162 $23,408 
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Stock options$6,842 $7,512 $13,825 $14,345 
ESPP341 290 663 591 
RSUs6,380 4,801 11,674 8,472 
$13,563 $12,603 $26,162 $23,408 
The following table summarizes option activity under our stock plans and related information:
Number of
Shares Subject
to Outstanding
Options		Weighted
Average
Exercise
Price
(Per
Share)		Weighted
Average
Remaining
Contractual
Term
(in years)		Aggregate
Intrinsic
Value
(in thousands)
Balance at December 31, 202210,082,642$29.12 6.30$27,141 
Options granted1,941,412$30.65 
Options forfeited(269,747)$33.95 
Options exercised(179,391)$8.92 
Balance at June 30, 202311,574,916$29.58 6.41$20,561 
Exercisable7,492,446$28.17 5.04$20,504 
We calculate the intrinsic value as the difference between the exercise price of the options and the closing price of common stock of $24.97 per share as of June 30, 2023.
The weighted-average fair value of options granted during the six-month periods ended June 30, 2023 and June 30, 2022 were $16.31 and $15.64 per share, respectively. There were 1,883,951 options granted during the six-month period ended June 30, 2022. We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following weighted average assumptions for the three and six months ended June 30, 2023 and 2022:
OptionsOptions
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Expected term (years)6.56.36.16.4
Expected volatility51.2 %53.1 %50.5 %53.0 %
Risk-free interest rate3.69 %3.13 %4.17 %1.93 %
Expected dividend yield— %— %— %— %
ESPPESPP
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Expected term (years)
0.5 - 2.0
0.5 - 2.0
0.5 - 2.0
0.5 -2.0
Expected volatility
38.2% - 55.7%
43.2% - 55.7%
38.2% - 55.7%
43.2% - 55.7%
Risk-free interest rate
0.13% - 5.39%
0.13% - 2.82%
0.13% - 5.39%
0.13% - 2.82%
Expected dividend yield— %— %— %— %
As of June 30, 2023, the unamortized compensation expense related to unvested stock options was $66.3 million. The remaining unamortized compensation expense will be recognized over the next 2.7 years. As of June 30, 2023, the unamortized compensation expense under our ESPP was $0.6 million. The remaining unamortized expense will be recognized over the next 0.4 years.
The following table summarizes the RSU activity for the six-month period ended June 30, 2023:
Restricted
Stock
Units		Weighted
Average Grant
Date Fair Value
(Per unit)
Unvested RSUs at December 31, 20221,232,551$32.41 
Granted944,72630.82 
Vested(367,647)32.73 
Forfeited(75,672)31.93 
Unvested RSUs at June 30, 20231,733,958$31.50 
As of June 30, 2023, the unamortized compensation expense related to unvested RSUs was $43.3 million. The remaining unamortized expense will be recognized over the next 2.2 years.