BC is classified according to the expression of the estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 into five subtypes: Luminal A, Luminal B (HER2-positive or HER2-negative), HER2-positive, and triple-negative breast cancer (TNBC) (Harbeck et al., 2013). This evidence concerns the gene ESR1 and breast cancer.