ESR1 and cancer: In addition to the conventional chemotherapy agents, such as alkylating drugs, antimetabolites, plant alkaloid, and antineoplastic antibiotics, hormone-receptor-positive tumors benefit from endocrine treatments such as selective estrogen receptor modulators, e.g., tamoxifen or aromatase inhibitors, whereas HER2-positive cancers require targeted therapy with agents such as monoclonal antibodies against HER2, e.g., trastuzumab.