ESR1 and breast cancer: • Adjuvant treatment for ER + breast cancers (Early Breast Cancer Trialists' Collaborative Group., 1998). • Metastatic Breast Cancer (Lipton, 1982). • Ductal Carcinoma in Situ (Allred et al., 2012). • Reduction in Breast Cancer Incidence in High Risk Women (Visvanathan, 2009).