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Stockholders' Equity | 12 Months Ended
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Dec. 31, 2021
Stockholders' Equity |
Stockholders' Equity | Note 10. 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, 2021 and 2020. 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, 2021, we had a total of 10,794,273 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 and restated in May 2021, for issuance of common stock to employees, non-employee directors, consultants, and scientific advisors. Options are granted to employees, consultants, and scientific advisors under the 2010 Stock Plan, pursuant to a formula determined by our Board of Directors. All options are exercisable at the fair market value of the stock on the date of grant. Non-employee director options expire after ten years. In May 2021, our stockholders approved an increase in the number of shares of common stock reserved for issuance under the 2010 Stock Plan from 44,453,475 to 53,953,475. Option activity under the 2010 Stock Plan was as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Shares Subject to | ​
​ | ​ | Outstanding Options | ​
​ | ​ | ​ | ​ | Weighted Average | ​
​ |  | Shares |  | Exercise Price |
Balance at December 31, 2020 |  | 12,115,288 | ​ | $ | 88.31 | ​
Options granted |  | 2,540,313 | ​ | $ | 84.92 | ​
Options exercised |  | (622,944) | ​ | $ | 66.44 | ​
Options cancelled | ​ | (1,269,197) | ​ | $ | 91.44 | ​
Balance at December 31, 2021 |  | 12,763,460 | ​ | $ | 88.39 | ​
​ 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 7-year terms and vested over three years, with 33% vesting after one year and the remainder vesting in 24 equal monthly installments. Options to purchase a total of 8,024,951, 6,732,942 and 6,896,492 shares as of December 31, 2021, 2020 and 2019, respectively, were exercisable. The aggregate intrinsic value of options exercised for the years ended December 31, 2021, 2020 and 2019 were $12.7 million, $87.5 million and $113.8 million, respectively. At December 31, 2021, the aggregate intrinsic value of options outstanding and vested options are $13.5 million and $13.4 million, respectively. The following table summarizes information about stock options outstanding as of December 31, 2021 under the 2010 Stock Plan:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options Outstanding | ​ | Options Exercisable | ​
​ |  | ​ |  | Weighted Average |  | Weighted |  | ​ |  | Weighted | ​
​ | ​ | ​ | ​ | Remaining | ​ | Average | ​ | ​ | ​ | Average | ​
​ | ​ | Number | ​ | Contractual Life | ​ | Exercise | ​ | Number | ​ | Exercise | ​
Range of Exercise Prices | ​ | Outstanding | ​ | (in years) | ​ | Price | ​ | Exercisable | ​ | Price | ​
$22.05 - $68.62 |  | 1,304,595 | ​ | 6.05 | ​ | $ | 64.71 |  | 1,048,050 | ​ | $ | 64.04 | ​
$68.82 - $69.35 |  | 89,030 | ​ | 8.35 | ​ |  | 69.15 |  | 31,252 | ​ |  | 68.95 | ​
$72.27 - $72.27 |  | 1,470,150 | ​ | 6.88 | ​ |  | 72.27 |  | 770,726 | ​ |  | 72.27 | ​
$72.58 - $80.50 |  | 1,614,398 | ​ | 5.77 | ​ |  | 77.49 |  | 1,131,015 | ​ |  | 76.77 | ​
$80.56 - $83.58 |  | 1,338,974 | ​ | 9.26 | ​ |  | 83.34 |  | 72,885 | ​ |  | 81.68 | ​
$83.83 - $85.01 |  | 1,371,805 | ​ | 6.14 | ​ |  | 84.52 |  | 1,008,722 | ​ |  | 84.42 | ​
$85.34 - $94.63 |  | 1,655,056 | ​ | 7.63 | ​ |  | 91.18 |  | 927,332 | ​ |  | 92.12 | ​
$95.34 - $101.79 | ​ | 1,293,897 | ​ | 3.88 | ​ | ​ | 96.28 | ​ | 876,609 | ​ | ​ | 96.33 | ​
$102.65 - $113.64 | ​ | 1,853,430 | ​ | 6.11 | ​ | ​ | 109.99 | ​ | 1,386,235 | ​ | ​ | 111.21 | ​
$115.19 - $138.52 | ​ | 772,125 | ​ | 4.93 | ​ | ​ | 128.71 | ​ | 772,125 | ​ | ​ | 128.71 | ​
​ |  | 12,763,460 | ​ | ​ | ​ | ​ | ​ |  | 8,024,951 | ​ | ​ | ​ | ​
​ Restricted Stock Units and Performance Shares In 2014, we began granting restricted stock units (“RSUs”) and performance shares (“PSUs”) 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, 2021, 2020 and 2019, we recorded $8.3 million, $13.9 million and $9.9 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, 2020 |  | 3,284,583 | ​ | $ | 87.42 | ​
RSUs granted | ​ | 2,102,386 | ​ | $ | 82.39 | ​
PSUs granted | ​ | 214,449 | ​ | $ | 74.26 | ​
RSUs released | ​ | (857,367) | ​ | $ | 87.82 | ​
PSUs released | ​ | (200,353) | ​ | $ | 68.08 | ​
RSUs cancelled | ​ | (409,333) | ​ | $ | 89.17 | ​
PSUs cancelled | ​ | (167,477) | ​ | $ | 72.48 | ​
Balance at December 31, 2021 | ​ | 3,966,888 | ​ | $ | 84.91 | ​
​ The following table summarizes our shares available for grant under the 2010 Plan. Each RSU and PSU grant reduces the available share pool by 2 shares.
​ | ​ | ​ | ​
​ | ​ | Shares Available | ​
​ |  | for Grant |
Balance at December 31, 2020 |  | 5,515,182 | ​
Additional authorization |  | 9,500,000 | ​
Options, RSUs and PSUs granted |  | (7,174,371) | ​
Options, RSUs and PSUs cancelled | ​ | 2,272,487 | ​
Balance at December 31, 2021 |  | 10,113,298 | ​
​ Employee Stock Purchase Plan. On May 21, 1997, our stockholders adopted the 1997 Employee Stock Purchase Plan, which was most recently amended in April 2020 (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. We issued 264,503, 258,453 and 239,590 shares under the ESPP in 2021, 2020 and 2019, respectively. For the years ended December 31, 2021, 2020 and 2019, we recorded stock compensation expense of $4.6 million, $4.6 million and $3.4 million, respectively, as the ESPP is considered compensatory under the FASB stock compensation rules. As of December 31, 2021, 680,975 shares remain available for issuance under the ESPP.

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