In HER2+ but not HER2− tumours, baseline ESR1 expression was significantly correlated with the change in Ki67 levels, and while those ESR1 levels were suppressed in HER2− tumours, they were not significantly changed in HER2+ tumours; expression of oestrogen-regulated genes and ER-related gene modules was also changed less in HER2+ than in HER2− tumours. This evidence concerns the gene ESR1 and neoplasm.