ESR1 and breast cancer: The distribution of HER2‐E sub‐type and activity of HER2 oncogenic signalling seem to be heavily influenced by ER status, with HER2‐E sub‐type representing only 30% of molecular sub‐types within HER2‐positive/HR‐positive BC [44, 46, 48, 53, 54], some authors even claimed that HER2‐E sub‐type is predominantly ER‐negative [53].