TP53 and mismatch repair cancer syndrome 1: Overall the most frequent mutational signatures detected from pre-treatment tumour biopsy data (n = 22), in descending order, were Breast C (n = 14) and Breast B (n = 12) (both APOBEC mutagenesis associated), Breast A (n = 10) (mismatch repair deficiency (MMRd)), Breast J (n = 10) (Ageing associated), Breast K (n = 10)(homologous recombination repair deficiency (HRD)), Breast F (n = 9) (aetiology unknown) and Breast G (n = 7) (TP53 mutation) (Fig. 4a).