ESR1 and neoplasm: To gain better insights into the proteomic alterations in cellular signalling that occur upon ERα loss in metastatic BCa, we performed whole proteome and phospho‐proteome analyses of 47 metastatic BCa tumours obtained from 37 metastatic BCa patients, including 15 of ERα‐P, 14 ERα‐C and 18 ERα‐N (triple‐negative BCa cases).