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Restructuring | 6 Months Ended
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Jun. 30, 2025
Restructuring and Related Activities [Abstract] |
Restructuring | Restructuring In April 2023, in light of then-current market conditions, the Board of Directors took steps to extend the Company’s cash runway by pausing further development of the Company’s product candidate, SEL-302, for the treatment of methylmalonic acidemia, and conducting a targeted headcount reduction. On August 17, 2023, the Company announced additional steps to extend cash runway and maximize value for stockholders by continuing to prioritize development of the Company’s product candidate, SEL-212, and support of its collaboration with Astellas for Xork, and pausing further development of all of the Company’s other clinical and preclinical product candidates that it was no longer actively advancing. As a result of these measures, the Company implemented a restructuring plan that resulted in an approximate 90% reduction of the Company’s headcount as of April 2023. The following table summarizes the change in the Company’s accrued restructuring balance included in accrued expenses and other current liabilities on its consolidated balance sheets (in thousands):
| December 31, 2023 |  | Charges |  | Cash Payments |  | June 30, 2024
Severance liability | $ | 3,896 |  |  | $ | 805 |  |  | $ | (4,527) |  |  | $ | 174 |
| December 31, 2024 |  | Charges |  | Cash Payments |  | June 30, 2025
Severance liability | $ | 80 |  |  | $ | — |  |  | $ | (80) |  |  | $ | — |
The Company recognized restructuring expenses consisting of one-time cash severance payments and other employee-related costs. The Company recorded these restructuring charges based on each employee’s role to the respective research and development and general and administrative operating expense categories on its consolidated statements of operations and comprehensive income (loss). For the six months ended June 30, 2025, the Company recognized no restructuring charges. For the six months ended June 30, 2024, the Company recognized $0.2 million in research and development expenses and $0.6 million in general and administrative expenses. Payments for the restructuring plan were completed in the first quarter of 2025.

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- Definition The entire disclosure for restructuring and related activities. Description of restructuring activities such as exit and disposal activities, include facts and circumstances leading to the plan, the expected plan completion date, the major types of costs associated with the plan activities, total expected costs, the accrual balance at the end of the period, and the periods over which the remaining accrual will be settled. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 420 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 5.P.4.e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479823/420-10-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 420 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482017/420-10-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 420 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482017/420-10-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 420 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org/420/tableOfContent Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 420 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482017/420-10-50-1 + Details
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