To compensate for the limited prognostic and predictive power of the histopathological classification of breast carcinomas, Perou et al. introduced a new method of clustering breast tumors depending on their gene expression profiles and the presence or absence of estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor receptor 2 (HER2). This evidence concerns the gene ESR1 and breast neoplasm.