ESR1 and breast cancer: Most commonly known breast cancer biomarkers include cancer antigens (CA); CA 15-3, CA 27-29, protein-6 (CA-6), Human epidermal growth factor receptor 2 (HER2), estrogen receptors (ER), progesterone receptors (PR), circulating tumor cells (CTC), cluster of differentiation-44, 47 (CD44, CD47), mesenchymal-epithelial transition factor receptor (MET), breast cancer genes-1/-2 (BRCA1/2), carcinoembryonic antigen (CEA) [5], and tumor Protein 53 [6].