On the basis of gene expression patterns, the following molecular subtypes of BC have been identified: (i) luminal-like tumors (the most common subtype of BCs, showing high genetic expression of the estrogen receptor (ER) and lacking significant expression of Erb-B2); (ii) HER2-enriched tumors (characterized by overexpression of Erb-B2 and showing low genetic expression of ER); (iii) basal-like tumors (characterized by expression of genes found in the basal cells of the terminal duct lobular units) [2]. Here, ESR1 is linked to breast cancer.