ESR1 and breast cancer: Breast cancer can be classified into four distinct types based on immunohistochemical markers: endocrine-receptor-positive (estrogen or progesterone receptor, ER+ or PR+), tumors that express estrogen or progesterone receptors; human epidermal growth factor receptor 2-positive (HER2), tumors with overexpression of the human epidermal growth factor receptor 2; triple-positive, tumors that are positive for estrogen, progesterone, and HER2 receptors; and triple-negative, tumors that lack estrogen, progesterone, and HER2 receptors [6].