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Background  Cytidine base editors (CBEs), composed of a cytidine deaminase fused to Cas9 nickase (nCas9), enable efficient C-to-T conversion in various organisms. However, current base editors can induce unwanted bystander C-to-T conversions when multiple Cs are present in the ~ 5-nucleotide activity window of cytidine deaminase, which negatively affects their precision. Here, we develop a new base editor which significantly reduces unwanted bystander activities.    Results  We used an engineered human APOBEC3G (eA3G) C-terminal catalytic domain with preferential cytidine-deaminase activity in motifs with a hierarchy CC C >C C C>C C  (where the preferentially deaminated C is underlined), to develop an eA3G-BE with distinctive C C  context-specificity and reduced generation of bystander mutations. Targeted editing efficiencies of 18.3–58.0% and 54.5–92.2% with excellent C C  context-specificity were generated in human cells and rabbit embryos, respectively. In addition, a base editor that can further recognize relaxed NG PAMs is achieved by combining hA3G with an engineered SpCas9-NG variant. The A3G-BEs were used to induce accurate single-base substitutions which led to nonsense mutation with an efficiency of 83–100% and few bystander mutations in Founder (F0) rabbits at  Tyr  loci.    Conclusions  These novel base editors with improved precision and C C  context-specificity will expand the toolset for precise gene modification in organisms.

Precise base editing with CC context-specificity using engineered human APOBEC3G-nCas9 fusions