The classic pathological classification of breast cancer is based on the expression of hormone receptors and human epidermal growth factor receptor 2 (HER2): luminal breast cancer (estrogen receptor and progesterone receptor, ER/PR-positive), HER2-positive (HER2-positive, ER/PR-negative), and triple negative (ER-, PR-, HER2-negative) breast cancer (TNBC). This evidence concerns the gene ESR1 and breast cancer.