ESR1 and metastatic neoplasm: ERα‐P and ERα‐C were characterised by expression of ERα (50–100% ERα‐positive tumour‐cells) in primary disease, and by maintenance (5–100% ERα‐positive tumour cells, average = 58%) or loss (< 1% ERα‐positive cells) of ERα‐expression, respectively, in metastatic disease (Table S1).