ESR1 and breast cancer: Molecular analyses allow dividing BC into three groups: luminal BC (expressing estrogen receptor /ER + / or progesterone receptor /PR + /), HER2-enriched BC (overexpressing human epidermal growth factor receptor 2 and/or having the HER2 gene amplified, without expression of ER or PR) and triple negative breast cancer (TNBC, ER, PR − , and HER2 −) [2, 3].