Mutational signature profiles from a previously published primary breast cancer study25,26, which included some pre-treatment HER2+ tumour biopsies (n = 19), were overall similar in composition and sample frequency to the study cohort described here except for the presence of mutational signature Breast I (Fig. 4c; Supplementary Figs. 5, 6a). Here, ERBB2 is linked to breast carcinoma.