CBE4 is a custom-engineered fusion protein consisting of APOBEC3A, nCas9 (Cas9 nickase), and a nuclear localization signal (NLS). Designed as a cytidine base editor, it enables programmable, site-specific C-to-T conversions in genomic DNA, without inducing double-strand breaks (DSBs). APOBEC3A is a single-stranded DNA-specific cytidine deaminase that catalyzes C-to-U (cytosine to uracil) deamination. In the cellular context, the resulting U•G mismatch is resolved by DNA replication or repair machinery, ultimately leading to a C•G to T•A substitution. nCas9 guides the editor to the target site via an sgRNA while minimizing off-target double-strand cleavage. The NLS ensures efficient nuclear import, enabling effective editing in mammalian cells. Together, these components form a high-efficiency, low-indel base editing tool suitable for diverse genome engineering applications.