We observed that whilst the APOBEC associated mutational signatures Breast B and Breast C are present at some level in tumours across all breast cancer tumour subtypes, Breast B (P = 0.023) and Breast C (P = 0.0014) are significantly more prevalent in ER−/HER2+ versus ER−/HER2-neg tumour subtype (Fig. 4d; Supplementary Fig. 6b; ANOVA test followed by post-hoc pairwise Wilcox test). This evidence concerns the gene ESR1 and neoplasm.