Our study showed that the isoforms uc003awn (APOBEC3A) and uc003awo (APOBEC3B) were independently associated with a higher burden of APOBEC-mutational signature in multiple cancer types, while such an association for the uc011aoc (APOBEC3A/B) was only observed in breast cancer. This evidence concerns the gene APOBEC3B and breast cancer.