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Stock-Based Awards
 12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Stock-Based Awards Stock-Based Awards
Equity Incentive Plans
The Company's equity incentive plans, the 2017 Equity Incentive Plan (the “2017 Plan”), and previously, the 2015 Stock Incentive Plan (the “2015 Plan”), reserve shares of common stock for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. Upon adoption of the 2017 Plan, no new awards or grants are permitted under the 2015 Plan. The 2015 Plan continues to govern restricted stock awards and option awards previously granted thereunder.
Awards granted under the 2017 Plan and 2015 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value of the Company's common stock on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.
The 2017 Plan provides that the number of shares reserved and available for issuance under the 2017 Plan will automatically increase each January 1, beginning on January 1, 2019, by the lesser of (i) 10.0 million shares, (ii) 5% of the outstanding shares on the last day of the immediately preceding fiscal year, or (iii) such number of shares determined by the administrator of the 2017 Plan. In January 2022, common stock available for issuance under the 2017 Plan was increased by approximately 6.1 million shares as a result of this automatic increase provision. As of December 31, 2022 and 2021, there were approximately 9.9 million and 7.0 million common shares available for the Company to grant under the 2017 Plan, respectively.
Stock Option Activity
The following table summarizes option award activity under the 2017 Plan and the 2015 Plan: 
Number of
Options
Weighted-
Average
Exercise Price
Weighted-
Average
remaining
contractual
life (years)
Aggregate
Intrinsic
Value (in thousands)
Balance at December 31, 202113,686,386 $24.33 6.76$333,011 
Granted2,255,840 43.06 
Exercised(720,904)14.59 
Forfeited(547,605)41.82 
Balance at December 31, 202214,673,717 $27.03 6.24$127,865 
Vested and expected to vest at December 31, 202213,083,022 $30.24 6.67$84,709 
Exercisable at December 31, 20229,886,659 $21.71 5.66$101,574 
Aggregate intrinsic value represents the difference between the fair value of the Company's common stock and the exercise price of outstanding options. The total intrinsic value of options exercised was $12.6 million, $47.9 million, and $43.8 million as of December 31, 2022, 2021 and 2020, respectively. During the years ended December 31, 2022, 2021, and 2020 the weighted-average grant-date fair value of the options vested was $23.96, $11.83, and $11.51 per share, respectively. The weighted-average grant date fair value of all options granted during the years ended December 31, 2022, 2021 and 2020 was $26.00, $41.30, and $15.67 per share, respectively.
Stock Options Granted to Employees with Service-Based Vesting
The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model, which requires various assumptions, including the fair value of the Company’s common stock, expected term, expected dividend yield, expected volatility, and the risk-free interest rate. The fair value of the Company’s common stock is based on the current market price, unless an adjustment is determined to be required, through discussion with senior management, due to material non-public information known by the Company at the time of grant. The expected volatility of the Company’s stock options is estimated using a combination of average historical stock price volatility of the Company's stock and that of comparable public companies within the biotechnology and pharmaceutical industry that are deemed to be representative of future stock price trends, since the Company does not have sufficient trading history to rely solely on the volatility of its common stock. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Management considers whether the Company is in possession of material non-public information at the time of grants when making certain estimates, including volatility and the fair value of the Company's common stock. The expected term of stock options represents the period that the Company’s stock-options are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). The risk-free interest rate is based on the implied yield currently available on U.S. treasury notes with terms approximately equal to the expected life of the option. The expected dividend rate is zero as the Company currently has no history or expectation of declaring cash dividends on the Company’s common stock.
The following assumptions were used in estimating the fair value of grants during the:
Year Ended December 31,
202220212020
Expected term (in years)
5.50 - 6.08
5.50 - 6.08
5.50 - 6.08
Volatility
65.1% - 66.3%
61.0% - 63.4%
65.2% - 67.1%
Risk-free interest rate
1.5% - 4.3%
0.5% - 1.3%
0.3% - 1.7%
Dividend yield———
Performance and Market Contingent Stock Options Granted to Employees
In August and November 2015, the Board of Directors granted approximately 1.6 million and 0.1 million shares of performance- and market- contingent awards to members of the senior management team, respectively. These awards have an exercise price of $0.68 per share.
These awards have two separate market triggers for vesting based upon either (i) the successful achievement of stepped target closing prices on a national securities exchange for 90 consecutive trading days later than 180 days after the Company’s initial public offering for its common stock, or (ii) stepped target prices for a change in control transaction. The first of these market triggers has been met. In the event the second market trigger is not achieved by the specified timelines, such awards will terminate with respect to that portion of the shares. The expense recognized associated with these performance- and market- contingent awards was $0.3 million of general and administrative expense during the year ended December 31, 2020. There was no expense recognized in the years ended December 31, 2022 and 2021.
The Company used a lattice model with a Monte Carlo simulation to value these stock options. This valuation methodology utilized the estimated fair value of the Company’s common stock on grant date and several key assumptions, including expected volatility of the Company’s stock price based on comparable public companies, risk-free rates of return and expected dividend yield.
Restricted Stock Activity
We grant restricted stock units ("RSUs") under the 2017 Plan. The fair value of restricted stock underlying the RSUs is determined based on the closing market price of the Company's common stock on the date of grant.
Aggregated information regarding RSUs granted under the Plan for the year ended December 31, 2022 is summarized below:
Number of UnitsWeighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 20212,629,980 $43.97 
Granted1,797,231 37.55 
Vested and released(837,096)40.30 
Forfeited(259,461)44.52 
Unvested and expected to vest at December 31, 20223,330,654 $41.39 
The aggregate intrinsic value of RSUs is calculated as the closing price per share of the Company's common stock on the last trading day of the fiscal period, multiplied by the number of RSUs expected to vest. The total intrinsic value of RSUs expected to vest was $92.6 million as of December 31, 2022. During the years ended December 31, 2021 and 2020 the weighted-average grant-date fair value of RSUs granted was $68.30 and $29.31, respectively. The total fair value of RSUs that vested during the years ended December 31, 2022, 2021, and 2020 was $29.4 million, $39.8 million, and $8.7 million, respectively.
Stock-Based Compensation Expense
The Company’s stock-based compensation expense was as follows (in thousands):
Year Ended December 31,
202220212020
Research and development$60,196 $50,036 $29,002 
General and administrative39,651 35,211 21,349 
Total$99,847 $85,247 $50,351 
As of December 31, 2022, total unamortized stock-based compensation expense was $197.6 million. The weighted-average period over which such stock-based compensation expense will be recognized is approximately 2.5 years.
There was no tax benefit realized related to awards vested or exercised during the years ended December 31, 2022 and 2021. For the year ended December 31, 2020 a tax benefit of $1.0 million was realized related to awards vested or exercised during the period. There is no tax benefit on total stock-based compensation expense for the years ended December 31, 2022, 2021 and 2020 since the company has recorded a full valuation allowance on all deferred tax assets.