ESR1 and breast cancer: Due to the lack of expression of the estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2), this type of breast cancer cannot be effectively treated by hormone and targeted therapies in the form of antiestrogens (e.g., tamoxifen) and monoclonal antibodies [e.g., trastuzumab (Herceptin)], respectively [36].