Histological and molecular classification of breast cancer have revealed at least four major subgroups: 1) basal-like: estrogen receptor-negative (ER-), progesterone receptor-negative (PR-), HER2-negative (HER2-); 2) luminal A: estrogen receptor-positive (ER+, low grade); 3) luminal B: ER+, high grade; 4) HER2-positive (HER2+)[1]. This evidence concerns the gene ESR1 and breast cancer.