As previously reported for pan-cancer analysis of APOBEC-induced mutations (Haradhvala et al. 2016; Morganella et al. 2016; Seplyarskiy et al. 2016), SBS13 mutations in BRCA1/2-proficient breast cancers associated strongly with deamination of cytidines in the lagging strand template. Here, BRCA1 is linked to cancer.