Breast cancer can be divided into four major groups based on estrogen receptor (ER), human epidermal growth factor 2 (HER2), Ki-67 index, and progesterone receptor (PR) expression: Luminal A (ER+/HER2-/Ki-67 low/PR+), Luminal B (ER+/HER2+/Ki-67 low/PR low), HER2-enriched (ER low/HER2+/Ki-67 high/PR low), and Triple-Negative Breast Cancer (TNBC, ER-/HER2-/Ki-67 high/PR-) [9]. This evidence concerns the gene ESR1 and breast cancer.