ESR1 and breast cancer: Despite much effort in recent years, the discovery of critical mediators of breast cancer metastasis which could also represent feasible targets for therapy is still a major challenge, particularly for the aggressive basal-like subtype tumors, the majority of which are classified as triple-negative: estrogen receptor-/progesterone receptor-/human epidermal growth factor receptor 2-negative (ER−/PR−/Her2−).