Interestingly, while the proportion of HER2-negative tumours was stable across different levels of ER expression, the relative proportions of HER2 0 and HER2-low BC varied substantially: 38% versus 23% respectively in ER 0%, 32% versus 21% in ER 1–9%, reaching 21% versus 49% respectively in ER ≥ 95% (Cochran-Armitage p < 0.001; Supplementary Fig. 2). This evidence concerns the gene ERBB2 and breast cancer.