Indeed, gene expression profiling studies have demonstrated that breast tumors can be divided into at least four clinically relevant molecular subtypes, each with distinct disease outcomes: the predominantly estrogen-receptor (ER)-negative, progesterone-negative, HER-2-negative, basal-like subtype, the HER2/neu-positive subtype and at least the two ER-positive, luminal-like A and B subtypes characterized by their differences in proliferation rate [11]–[12]. This evidence concerns the gene ESR1 and breast neoplasm.