Breast tumors frequently display A3B overexpression and APOBEC signature mutations (Alexandrov et al., 2013; Angus et al., 2019; Bertucci et al., 2019; Burns et al., 2013a; Burns et al., 2013b; Nik-Zainal et al., 2012; Nik-Zainal et al., 2016; Roberts et al., 2013). The gene discussed is APOBEC3B; the disease is breast neoplasm.