ESR1 and breast cancer: More than 15 years ago, the analysis of the gene expression profiling of BC tumors suggested a new molecular classification that divides breast carcinoma into different subtypes according to their expression: (i) luminal (expression of ER, ER regulatory genes, and/or normal luminal epithelial cells), (ii) HER2-positive (amplification and/or overexpression of the ERBB2 gene), (iii) basal (ER, PR, and HER2-negative, but with an expression of genes from normal mammary myoepithelial and basal cells), and (iv) normal-like subtype [16,17,18].