Based on the expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER-2) and Ki-67, breast cancers have been classified into Luminal A (HER2-/ER +/PR +, low proliferation), Luminal B (HER2-/ER +/PR +, high proliferation), HER2-amplified (HER2 +/ER + or PR + or ER +/PR +; HER2 +/ER-/PR-) and triple-negative breast cancer (TNBC) (HER2-/ER-/PR-) types (3). This evidence concerns the gene ESR1 and breast cancer.