Levels of the several different human apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3; A3) cytidine deaminases vary across different cell types. While levels of nuclear-localized A3s are low or non-existent in primary cells, a nuclear A3 is often upregulated in cancer cells. Increased chromosomal mutagenesis due to nuclear A3...