By investigating the presence of APOBEC signatures (SBS2/13) and signature SBS39 in breast cancer subtypes defined by TCGA36, we found a significant enrichment of SBS39 exposures in the basal subtype compared with 3 other breast cancer subtypes defined by PAM50 (Her2, Luminal A, Luminal B; Supplementary Fig. 5). Here, ERBB2 is linked to breast cancer.