a, Protospacer (black and red) and PAM (blue) sequences of the six mammalian cell genomic loci targeted by base editors. Target Cs are indicated in red with subscripted numbers corresponding to their positions within the protospacer. b, Synthetic 80-mers with sequences matching six different genomic sites were incubated with BE1 then analysed for base editing by high-throughput sequencing. For each site, the sequence of the protospacer is indicated to the right of the name of the site, with the PAM highlighted in blue. Underneath each sequence are the percentages of total DNA sequencing reads with the corresponding base. We considered a target C as ‘editable’ if the in vitro conversion efficiency is >10%. Note that maximum yields are 50% of total DNA sequencing reads since the non-targeted strand is unaffected by BE1. Values are shown from a single experiment. c, HEK293T cells were transfected with plasmids expressing BE1 and an appropriate sgRNA. Three days after transfection, genomic DNA was extracted and analysed by high-throughput sequencing at the six loci. Cellular C to T conversion percentages, defined as the percentage of total DNA sequencing reads with Ts at the target positions indicated, are shown for BE1 at all six genomic loci. Values and error bars of all data from HEK293T cells reflect the mean and standard deviation of three independent biological replicates performed on different days.