a, Base editing strategy. DNA with a target C (red) at a locus specified by a guide RNA (green) is bound by dCas9 (blue), which mediates local DNA strand separation. Cytidine deamination by a tethered APOBEC1 enzyme (red) converts the single-stranded target C→U. The resulting G:U heteroduplex can be permanently converted to an A:T base pair following DNA replication or DNA repair. b, Deamination assay showing a BE1 activity window of approximately five nucleotides. Samples were prepared as described in the Methods. Each lane is labelled according to the position of the target C within the protospace, or with ‘–’ if no target C is present, counting the base distal from the PAM as position 1. c, Deamination assay showing the sequence specificity and sgRNA-dependence of BE1. The DNA substrate with C at position 7 in b was incubated with BE1 and the correct sgRNA, a mismatched sgRNA or no sgRNA. The positive control sample used a synthetic DNA substrate with a U at position 7. For uncropped gel data, see Supplementary Figure 1.