ESR1 and neoplasm: Interestingly, in this study, analysis of mutational signatures in pre-treatment tumour biopsies pointed to a greater prevalence of APOBEC associated mutational signatures in HER2+ versus HER2-neg tumours overall but furthermore the relative contribution of APOBEC associated mutational signature, Breast C, analogous to COSMIC reference signature 13, was highest in ER-/HER2+ compared to ER−/HER2− clinical tumour subtypes.