ESR1 and breast cancer: Genomic studies have increased our understanding of the heterogeneity of BC, allowing its classification into four intrinsic subtypes of invasive breast cancer (IBC) based on receptor expression: luminal A, characterized by the expression of estrogen and/or progesterone receptors (ER/PR); luminal HER, characterized by the expression of ER and/or PR and human epidermal growth factor receptor 2 (HER-2); HER-2, characterized by HER-2 overexpression and absence of ER and PR expression; and the triple-negative (TNBC) subtype, which does not express any of these three receptors [29,30].