ESR1 and breast cancer: Elacestrant monotherapy is indicated by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) for the treatment of post-menopausal women and adult men with HER2-negative, ER-positive advanced breast cancer, with an ESR1-activating mutation who have disease progression after receiving at least one line of endocrine therapy including a CDK 4/6 inhibitor.