ESR1 and breast cancer: For the treatment choice, patients are classified according to intrinsic biological subtypes within the BC spectrum, using clinical-pathological criteria, i.e., the recognition of amplification and/or overexpression of the human epidermal growth factor receptor 2 (HER2) oncogene, the immunohistochemical classification of the estrogen receptor (ER) and the progesterone receptor (PR) and Ki-67 labeling index [2].