According to immunohistochemical markers, breast cancer has five subtypes that differ in prognosis and therapeutic targets: 1) luminal A (ER positive and/or PR positive and HER2 negative), 2) luminal B (estrogen receptor positive and/or progesterone receptor positive and HER2 positive), 3) HER2 overexpressing (estrogen receptor and progesterone receptor negative and HER2 positive), 4) basal like (estrogen receptor/progesterone receptor/HER2 negative, cytokeratin 5/6 positive, and/or epidermal growth factor receptor positive), and 5) normal breast like. This evidence concerns the gene ESR1 and breast cancer.