There is increasing evidence indicating that BC subtypes—characterized by the presence or absence of specific receptors on tumor cells: hormone receptors (HR) (estrogen receptors (ER) and progesterone receptors (PR)), and human epidermal growth factor receptor 2 (HER2)—show etiological differences [11] and distinct biological behaviors and prognostic outcomes [12], which might be partially influenced by the characteristics of the breast tissue in which they arise. This evidence concerns the gene ESR1 and breast cancer.