Kanu and colleagues focused on the association between replication stress and APOBEC3B activity in breast cancer cell lines, starting from the observation that the HER2-enriched subtype of breast cancers exhibits the greatest burden of APOBEC mutations [8] and the premise that oncogene-induced replication stress exposes single-stranded DNA (ssDNA) substrate susceptible to APOBEC mutagenesis [2]. The gene discussed is APOBEC3B; the disease is breast carcinoma.