On the basis of cellular gene expression profiles, 5 subtypes of breast cancer have been defined: luminal type A (ER+/progesterone receptor (PR)+/human epidermal growth factor receptor 2(HER2)-), luminal type B (ER+/PR+/HER2+), HER2-overexpression type (ER-/PR-/HER2+), basal-like type (ER-/PR-/HER2-), and normal-like type (the gene expression profile of cells is similar to that of normal breast epithelial cells, showing features of a low treatable rate via chemotherapy, a high quality prognosis, and a lower mortality rate if detected and treated early) [2,3]. This evidence concerns the gene ESR1 and breast cancer.