Base editors enable precise correction of point mutations without requiring DNA double-strand breaks, yet platform– and cell type–specific genotoxicities remain incompletely characterized. Here, we applied cytosine base editing (CBE) to disrupt a cryptic splice-site mutation in the Unc13d locus of Jinx mice, a model of familial hemophagocytic lymphohistiocytosis type 3 (FHL3). Efficient editing (62–89%) in fibroblasts, T cells, and hematopoietic stem cells (HSCs) restored Unc13d splicing, reconstituted cytotoxic T cell function, and protected mice from virus-triggered hyperinflammation after transplantation of edited HSCs. Comparative genotoxicity profiling revealed distinct platform- and cell type–specific patterns: hyperactive CBE induced broader off-target activity and more structural variants than CRISPR–Cas9, particularly in HSCs. While off-target sequence edits persisted, CBE-induced chromosomal translocations largely resolved in vivo. These findings establish therapeutic base editing for a genetically predisposed hyperinflammatory syndrome and underscore the importance of context-specific safety profiling to guide clinical translation of genome editors.

Murine T cells, HSCs and MEF cells were edited using Cas9 nuclease or evoCDA1-BE4max at the Unc13D locus. CAST-Seq was performed to assess the formation of chromosomal translocations between the on-target site and potential off-target sites.