To reveal the mechanisms underlying this heterogeneity, breast cancer has been classified into the following major molecular subtypes according to different gene expression profiles: normal breast-like, luminal A (ER+ or PR+/HER2−), luminal B (ER+ or PR+/HER2+), HER2-positive (HER2+), and triple negative/basal-like (ER−/PR−/HER2−) subtypes [5], [6], [7]. This evidence concerns the gene ESR1 and breast cancer.