Schematic of the basic structure of CRISPR DNA base editors. Cytosine base editors (CBE) consist of a nickase Cas9 variant (Cas9*) fused with a cytidine deaminase enzyme. In addition, one or two copies of an uracil glycosylase inhibitor (UGI) are fused with the protein complex to prevent cellular base excision repair (BER) mechanisms. CBEs introduce a C-to-T point mutation into DNA with a C:U intermediate. For adenine base editors (ABE) Cas9* is fused with an deoxyadenosine deaminase enzyme enabling a permanent A-to-G point mutation in DNA with a A:I intermediate. The target base is shown in the single-stranded R-loop. Diagram indicating the targeting window for CBE or ABE, with deamination positions between position 4–8 (CBE) and 4–7 (ABE) on the protospacer sequence, when the PAM site is counting as positions 20–23.