This heterogeneity was further unraveled by the molecular classification proposed by Perou et al. and Sørlie et al., who classified breast cancers into four groups: luminal breast cancer (characterized by the expression of ESR1, coding for ERα), HER2-enriched, basal-like breast cancers, and the controversial “normal breast-like” [6,7]. This evidence concerns the gene ESR1 and breast cancer.