According to several biomarkers, such as the expression of progesterone receptor (PgR) and the Ki67 labeling index (LI), and several genomic tests (i.e., Oncotype DX and MammaPrint), ER-positive and HER2-negative breast cancer is classified into the luminal A-like (low-risk group), luminal B-like (high-risk group), and intermediate types [2, 3]. This evidence concerns the gene ESR1 and breast cancer.