APOBEC3A and breast cancer: The ICGC breast tumor data set lacks sufficient A3B-null tumors for a robust SBS signature analysis but a retrospective analysis of 17 null tumors in the TCGA breast cancer cohort indicates a high proportion of APOBEC3 signature YTCW mutations indicative of (but perhaps not exclusively due to) A3A activity (67% YTCW from a total of n = 538 exomic SBS mutations) [46].