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Stock-based Compensation
3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-based Compensation Stock-based Compensation
Employee Stock Purchase Plan
In April 2023, the Board approved an employee stock purchase plan (the “ESPP”) which became effective upon approval at the Annual Meeting in June 2023. The ESPP allows eligible employees of the Company to purchase shares of our common stock through payroll deductions over 24-month offering periods. The per share purchase price will be the lower of 85% of the closing trading price per share of our common stock on the first trading date of an offering period in which a participant is enrolled or 85% of the closing trading price per share on the purchase date. Participant purchases are limited to a maximum of $25,000 of fair value of our stock per calendar year. The Company is authorized to grant up to 2,000,000 shares of common stock under the ESPP. The first offering period of the ESPP commenced in December 2023 and as of March 31, 2024, the Company had not issued any shares of common stock under the ESPP. We recognized $0.1 million of stock-based compensation expenses related to the ESPP in the three months ended March 31, 2024.
Stock Options

Stock options granted to employees generally have a maximum term of ten years and vest over four years from the date of grant, of which 25% vest at the end of one year, and 75% vest monthly over the remaining three years. In January 2024, the Board approved the grants of stock options with a vesting term over three years from the date of grant, of which 33% vest at the end of one year, and 67% vest monthly over the remaining two years.
Stock-Based Compensation Expense
Stock-based compensation expense is included in the unaudited condensed consolidated statements of operations as follows (in thousands):
 Three Months Ended March 31,
 20242023
Cost of product revenue$115 $129 
Research and development 820 722 
Selling, general and administrative2,236 1,958 
Total$3,171 $2,809 
The following table presents total stock-based compensation expense by security type included in the unaudited condensed consolidated statements of operations (in thousands):
 Three Months Ended March 31,
 20242023
Stock options$1,514 $922 
RSUs and RSAs1,317 1,126 
PSUs247 837 
PBOs16 (76)
ESPP77 — 
Total$3,171 $2,809 
As of March 31, 2024, unrecognized stock-based compensation expense, net of expected forfeitures, was $13.3 million related to unvested stock options and $6.5 million related to unvested RSUs and RSAs. Stock-based compensation expense for these awards will be recognized through 2028.