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Summary of Stock-Based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] |  |  |  |  |
Total stock-based compensation expense |  | $ 713 | $ 1,068 | $ 2,722 | $ 2,707
Research and development |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] |  |  |  |  |
Total stock-based compensation expense |  | 158 | 493 | 921 | 1,151
General and Administrative |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] |  |  |  |  |
Total stock-based compensation expense | [1] | $ 555 | $ 575 | $ 1,801 | $ 1,556
[1] | Included in general and administrative for the three and nine months ended September 30, 2021 is stock-based compensation expense related to 15,961 shares and 39,823 shares, respectively, of common stock issued to certain board members in lieu of their cash compensation.

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