ESR1 and breast cancer: It should be also considered that BC is classified into four subtypes based on clinicopathologic features defined by immunohistochemical expression of estrogen receptor (ER) or progesterone receptor (PR) and human epidermal growth factor receptor 2 (Her2): ER/PR+, Her2+; ER/PR+, Her2−; ER/PR−, Her2+; and ER/PR−, Her2− (110).