Summary of plans and activity

In June 2001, our Board of Directors and stockholders established the 2001 Stock Incentive Award Plan (“2001 Plan”). Under the 2001 Plan, as amended, 2,674,749 shares of common stock had been reserved for the issuance of incentive stock options granted to employees, non-employee directors, consultants, or independent contractors. Options granted under the 2001 Plan have vesting terms that range from the date of grant to four years and expire within a maximum term of 10 years from the grant date.

In 2021, our Board of Directors and stockholders established the 2021 Equity Incentive Plan (“2021 Plan”). The number of shares of common stock initially reserved for issuance under the 2021 Plan was 1,854,490 newly reserved shares in addition to the 600,737 shares that remained available for issuance under the 2001 Plan. The shares available for issuance under the 2021 Plan automatically increase on the first day of each year, commencing January 1, 2022, and ending on (and including) January 1, 2031, in an amount equal to 5% of the total number of shares of our common stock outstanding on the last day of the calendar month before the date of each automatic increase, or such lesser number of shares as determined by the Board of Directors. The annual increase resulted in an additional 1,315,580 shares being reserved for issuance under the 2021 Plan as of January 1, 2026. The 2021 Plan provides for the issuance of stock options, stock appreciation rights, restricted stock awards, stock unit awards and other stock-based awards and cash incentive awards to employees, consultants and non-employee directors of the Company and its subsidiaries. Awards granted under the 2021 Plan will have such vesting schedules and other terms as determined by the Compensation Committee and stock options and stock appreciation rights have a maximum term of 10 years from the grant date. No further awards can be granted under the 2001 Plan following the adoption of the 2021 Plan.

As of March 31, 2026, there were 539,390 shares available for future issuance under the 2021 Plan, based on awards outstanding and reserved at target levels for PSUs. In connection with the PSUs granted during 2026, we evaluated the number of shares that could be issued assuming maximum achievement of the applicable performance conditions. If PSUs are reserved at the maximum payout level, the number of shares subject to outstanding awards could exceed the shares available under the plan as of March 31, 2026. We expect that we will have sufficient shares available for issuance under the 2021 Plan to satisfy these awards as a result of future automatic annual increases pursuant to the terms of the plan prior to the end of the performance period.

Stock Options

Options are granted at exercise prices not less than the fair market value of our common stock on the date of grant. Prior to our initial public offering (the “IPO”), the fair market value of our common stock was determined by our Board of Directors, and following our IPO, the fair market value of our common stock is based on the closing price of our common stock on the date of grant.

​

During the years 2008 through the IPO, the Board of Directors authorized the grant of stock options for the purchase of shares of common stock to the employers of certain non-employee directors. The options were not granted under the 2001 Plan or the 2021 Plan, but terms are substantially the same as our standard form of option agreement for non-employee directors as they have an exercise price not less than the fair market value on the grant date and vest over 48 months from the date of grant.

The following is a summary of stock option activity:

​

As of March 31, 2026, stock options outstanding included 4,363 options that were not granted under the 2001 Plan or the 2021 Plan. For options outstanding as of March 31, 2026, the weighted average remaining contractual life was 6.8 years. For options exercisable as of March 31, 2026, the weighted average remaining contractual life was 5.4 years. As of March 31, 2026, unrecognized compensation expense related to unvested stock-based compensation arrangements for stock options was $22.4 million. As of March 31, 2026, the related weighted average period over which the expense is expected to be recognized is approximately 2.8 years.

Restricted Stock Units

RSUs are share awards that entitle the holder to receive freely tradable shares of our common stock upon vesting. The RSUs cannot be transferred and the awards are subject to forfeiture if the holder’s service terminates prior to vesting other than for death, disability, or other qualifying terminations.

The following is a summary of RSU activity:

​

​

The aggregate intrinsic value of unvested RSUs is based on our closing stock price on the last trading day of the period. 80,660 RSUs were vested as of March 31, 2026. As of March 31, 2026, the unrecognized compensation expense related to unvested stock-based compensation arrangements for RSUs was $8.4 million. As of March 31, 2026, the related weighted average period over which the expense is expected to be recognized is approximately 3.6 years.

Performance Stock Units

We grant PSUs to officers and key employees. The number of PSUs that will ultimately be earned is based on our performance relative to pre-established performance goals for the respective performance period. The expense is recorded on a straight-line basis over the requisite performance and service periods based on an estimate of the number of PSUs expected to vest. Management expectations related to the achievement of the performance goals associated with PSU grants are assessed each reporting period. The number of PSUs

earned at the end of the performance period ranges from 0% to 200% of the number of PSU’s granted. The PSUs cannot be transferred and the awards are subject to forfeiture if the holder’s service terminates prior to vesting other than for death, disability, or other qualifying terminations.

If the performance conditions are not met or not expected to be met, any compensation expense recognized associated with the grant will be reversed.

The following is a summary of PSU activity:

​

​

The aggregate intrinsic value of unvested PSUs is based on our closing stock price on the last trading day of the period. As of March 31, 2026, there was $4.7 million of unrecognized stock-based compensation expense related to outstanding PSUs that is expected to be recognized over a weighted-average period of 2.3 years.

Employee Stock Purchase Plan

Our Board of Directors and stockholders also established an Employee Stock Purchase Plan (the “ESPP”). The number of shares of common stock initially reserved for issuance under the ESPP was 278,170. The shares available for issuance under the ESPP automatically increase on the first day of each year, commencing January 1, 2022, and ending on (and including) January 1, 2031, in an amount equal to 1% of the total number of shares of our common stock outstanding on the last day of the calendar month before the date of each automatic increase, or such lesser number of shares as determined by the Board of Directors. No annual increase was made to the shares reserved for issuance under the ESPP as of January 1, 2026. The ESPP permits certain of our U.S. employees to purchase shares of our common stock at a price per share not less than 85% of the lower of (i) the closing market price per share of our common stock on the first day of the applicable purchase period or (ii) the closing market price per share of our common stock on the purchase date at the end of the applicable six-month purchase period. For the three months ended March 31, 2026, there were no shares of common stock purchased under the ESPP. As of March 31, 2026, there were 737,396 shares available for issuance under the ESPP.

Stock-based compensation expense

We use the Black-Scholes option pricing model to determine the fair value of stock options and ESPP purchase rights on the grant date. The fair values of RSUs and PSUs are determined based on the closing stock price of our common stock on the grant date. We measure stock-based compensation expense based on the grant date fair value of the award and recognize compensation expense over the requisite service period, which is generally the vesting period for stock options, RSUs, and PSUs, and the offering period for ESPP purchase rights. The amount of stock-based compensation expense recognized for stock option, RSU and PSU awards during a period is based on the portion of the awards that are ultimately expected to vest. The amount of stock-based compensation expense recognized for ESPP purchase rights during a period is based on the estimated purchase rights as of the grant date. We account for forfeitures as they occur.

The following table provides the weighted average fair value of options granted to employees and the related assumptions used in the Black-Scholes option pricing model for the three months ended March 31, 2026 and 2025:

​

​

The following table provides the weighted average fair value of ESPP purchase rights and the related assumptions used in the Black-Scholes option pricing model for the three months ended March 31, 2026 and 2025:

​

​

We review these assumptions on a periodic basis and adjust them, as necessary. We utilize the simplified method to develop the estimate of the expected term for stock option awards and ESPP purchase rights. The expected volatility is based upon our historical stock price. The expected dividend yield is assumed to be zero, as we have never paid dividends and have no current plans to do so. The risk-free interest rate is based on the yield on U.S. Treasury securities for a period approximating the expected term of the options being valued.

The following table presents the components and classification of stock-based compensation expense for the periods indicated:

​