ESR1 and breast cancer: The biomarkers involved in BC include immunohistochemical markers [estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor-2 (HER2) and the proliferation marker protein Ki-67], genomic markers [BReast CAncer gene 1 and 2 (BRCA1, BRCA2 and Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Alpha (PIK3CA)] as well as immunomarkers [tumor-infiltrating lymphocytes and Programmed Death Ligand-1 (PD-L1)] (5).