Accordingly, three basic therapeutic groups [32] have been identified: (i) the ER positive group, also named “luminal group” and, further divided into luminal A and luminal B, is the most numerous and diverse in terms of gene expression and mutation spectrum; (ii) the HER2 (also called erb-b2 receptor tyrosine kinase 2, ERBB2) amplified group; (iii) the triple-negative breast cancers (TNBCs), also known as “basal-like” breast cancers, characterized by lacking expression of ER, PR and HER2, more frequent in patients with germline BRCA1 mutations. This evidence concerns the gene ESR1 and breast cancer.