Although A3B was thought to be primarily responsible for APOBEC-induced mutagenesis for most tumor types (Burns et al. 2013a,b), recent studies indicate A3A is the predominate source of APOBEC-induced mutation in breast cancer tumors and potentially in other tumor types (Chan et al. 2015; Cortez et al. 2019; Petljak et al. 2022). This evidence concerns the gene APOBEC3A and breast cancer.