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Stockholders' Equity | 12 Months Ended
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Dec. 31, 2023
Stockholders' Equity Note [Abstract] |
Stockholders' Equity | Note 11. Stockholders' Equity Preferred Stock. We are authorized to issue 5,000,000 shares of preferred stock, none of which was outstanding as of December 31, 2023 and 2022. The Board of Directors may determine the rights, preferences and privileges of any preferred stock issued in the future. Common Stock. We are authorized to issue 400,000,000 shares of common stock. Stock Compensation Plans. As of December 31, 2023, we had a total of 11,557,443 shares of our common stock available for future issuance related to our stock plans as described below. 2010 Stock Incentive Plan. In May 2010 the Board of Directors adopted the 2010 Stock Incentive Plan (the “2010 Stock Plan”), which was most recently amended in April 2023, for issuance of common stock to employees, non-employee directors, consultants, and scientific advisors. Awards under the 2010 Stock Plan include stock options, RSUs and PSUs. In June 2023, our stockholders approved an increase in the number of shares of common stock reserved for issuance under the 2010 Stock Plan from 53,953,475 to 66,453,475. Stock Options Options are granted to employees, consultants, and scientific advisors under the 2010 Stock Plan. Options are also granted under the 2010 Stock Plan to non-employee members of our Board of Directors, pursuant to a formula set forth in the 2010 Stock Plan. All options are exercisable at the fair market value of the stock on the date of grant. In July 2016, we revised the terms of our annual stock option grants to provide that new option grants would generally have a 10-year term and vest over four years, with 25% vesting after one year and the remainder vesting in 36 equal monthly installments. Previously, our option grants generally had seven-year terms and vested over three years, with 33% vesting after one year and the remainder vesting in 24 equal monthly installments. Non-employee director options expire after 10 years and vest in full on the first anniversary of the date of grant or, if earlier, the date of the next annual meeting of stockholders. Option activity under the 2010 Stock Plan was as follows:
| Shares Subject to Outstanding Options
| Shares |  | Weighted Average Exercise Price
Balance at December 31, 2022 | 12,650,359 |  | $ | 87.25 |
Options granted | 1,443,064 |  | $ | 69.60 |
Options exercised | (231,417) |  | $ | 67.05 |
Options cancelled | (1,404,848) |  | $ | 88.87 |
Balance at December 31, 2023 | 12,457,158 |  | $ | 85.40 |
Options to purchase a total of 9,743,775, 8,952,289 and 8,024,951 shares as of December 31, 2023, 2022 and 2021, respectively, were exercisable. The aggregate intrinsic value of options exercised for the years ended December 31, 2023, 2022 and 2021 were $3.2 million, $6.0 million and $12.7 million, respectively. At December 31, 2023, the aggregate intrinsic value of options outstanding and vested options are $2.0 million and $1.9 million, respectively. The following table summarizes information about stock options outstanding as of December 31, 2023 under the 2010 Stock Plan:
|  | Options Outstanding |  | Options Exercisable
Range of Exercise Prices |  | Number Outstanding |  | Weighted Average Remaining Contractual Life (in years) |  | Weighted Average Exercise Price |  | Number Exercisable |  | Weighted Average Exercise Price
$49.01 - $68.62 |  | 1,752,776 |  | 6.94 |  | $ | 64.26 |  | 904,749 |  | $ | 67.01
$68.70 - $72.27 |  | 1,279,346 |  | 5.25 |  | 71.94 |  | 1,207,361 |  | 72.08
$72.66 - $76.95 |  | 1,271,472 |  | 7.27 |  | 74.75 |  | 789,924 |  | 74.69
$77.03 - $80.50 |  | 1,310,294 |  | 6.69 |  | 79.18 |  | 945,947 |  | 79.64
$80.56 - $83.58 |  | 1,502,140 |  | 7.63 |  | 83.26 |  | 873,400 |  | 83.23
$83.83 - $85.52 |  | 1,269,328 |  | 4.36 |  | 84.61 |  | 1,222,325 |  | 84.60
$85.60 - $94.63 |  | 1,269,423 |  | 5.82 |  | 91.66 |  | 1,097,345 |  | 92.06
$95.34 - $106.47 |  | 1,326,730 |  | 5.03 |  | 101.65 |  | 1,227,075 |  | 101.33
$107.69 - $128.34 |  | 1,332,485 |  | 2.98 |  | 118.11 |  | 1,332,485 |  | 118.11
$132.00 - $138.52 |  | 143,164 |  | 2.83 |  | 134.76 |  | 143,164 |  | 134.76
|  | 12,457,158 |  |  |  |  |  | 9,743,775 |  |
Restricted Stock Units and Performance Shares RSUs and PSUs are granted to our employees at the share price on the date of grant. Each RSU represents the right to acquire one share of our common stock. Each RSU granted in connection with our annual equity awards will vest 25% annually over four years, while each RSU granted as outstanding merit awards or as part of retention award programs will vest in a single installment at the end of four years. We grant PSUs with performance and/or service-based milestones with graded and/or cliff vesting over three to four years. The shares of our common stock into which each PSU may convert is subject to a multiplier based on the level at which the financial, developmental and market performance conditions are achieved over the service period. Compensation expense for PSUs with financial and developmental performance conditions is recorded over the estimated service period for each milestone when the performance conditions are deemed probable of achievement. For PSUs containing performance conditions which were not deemed probable of achievement, no stock compensation expense is recorded. Compensation expense for PSUs with market performance conditions is calculated using a Monte Carlo simulation model as of the date of grant and recorded over the requisite service period. For the years ended December 31, 2023, 2022 and 2021, we recorded $17.2 million, $7.8 million and $8.3 million, respectively, of stock compensation expense for PSUs on our consolidated statements of operations. RSU and PSU award activity under the 2010 Stock Plan was as follows:
| Shares Subject to Outstanding Awards
| Shares |  | Grant Date Value
Balance at December 31, 2022 | 5,187,592 |  | $ | 81.24 |
RSUs granted | 3,576,242 |  | $ | 65.26 |
PSUs granted | 300,512 |  | $ | 61.76 |
RSUs released | (1,264,781) |  | $ | 83.43 |
PSUs released | (108,712) |  | $ | 102.44 |
RSUs cancelled | (412,446) |  | $ | 76.61 |
PSUs cancelled | (113,065) |  | $ | 70.57 |
Balance at December 31, 2023 | 7,165,342 |  | $ | 72.17 |
The following table summarizes our shares available for grant under the 2010 Plan. E ach RSU and PSU grant reduces the available share pool by 2 shares.
| Shares Available for Grant
Balance at December 31, 2022 | 5,056,370
Additional authorization | 12,500,000
Options, RSUs and PSUs granted | (9,196,930)
Options, RSUs and PSUs cancelled | 2,455,586
Balance at December 31, 2023 | 10,815,026
Employee Stock Purchase Plan. On May 21, 1997, our stockholders adopted the 1997 Employee Stock Purchase Plan, which was most recently amended in April 2023 (the “ESPP”). Each regular full-time and part-time employee working 20 hours or more per week is eligible to participate after one month of employment. In June 2023, our stockholders approved an increase in the number of shares of common stock reserved for issuance under the ESPP from 9,600,000 to 10,350,000. We issued 380,145, 308,413 and 264,503 shares under the ESPP in 2023, 2022 and 2021, respectively. For the years ended December 31, 2023, 2022 and 2021, we recorded stock compensation expense of $5.1 million, $4.7 million and $4.6 million, respectively, as the ESPP is considered compensatory under the FASB stock compensation rules. As of December 31, 2023, 742,417 shares remain available for issuance under the ESPP.

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