The most common classification of breast cancer is based on the expression of hormone receptors (estrogen receptor (ER) and progesterone receptor (PR)) and human epidermal growth factor receptor 2 (HER2), grouping it into luminal A (positive for ER, PR, low Ki67), luminal B (positive for all, ER, PR, high Ki67), luminal B HER2-positive (positive for ER, PR, and HER2, high Ki67), HER2-enriched which is negative for ER and PR, and triple-negative breast cancer (TNBC) (negative for all, ER, PR, and HER2) [2]. This evidence concerns the gene ESR1 and breast cancer.