BE3 is an artificial 'base editor' that can be used to irreversibly convert a single target cytidine DNA base in a programmable manner, without requiring double-stranded cleavage of the DNA backbone or a donor template; it favors the conversion of C to T. BE3 is composed of the rat Apobec1 cytidine deaminase ( RGDID:2133 ) fused, via a 16-residue XTEN linker, to the Cas9(D10A) nickase, followed by the uracil DNA glycosylase inhibitor (UGI) from Bacillus subtilis bacteriophage PBS1. The Cas9(D10A) sequence allows BE3 to be targeted to a genomic target site of interest in the presence of a guide RNA (gRNA), where the APOBEC1 enzyme mediates the conversion of cytidine to uridine (conversion is seen within a window of approximately 5 nucleotides), resulting in a U:G mismatch which can potentially be resolved in a number of ways. The presence of the UGI sequence in BE3 inhibits base-excision repair by cellular uracil DNA glycosylase, preventing reversion of the U to C, while Cas9(D10A) nicks the non-edited strand containing the G residue opposite the edited U, favoring mismatch repair of the nicked strand. These properties favor the conversion of the targeted C base to T (PMID:27096365).