ESR1 and breast cancer: The therapeutic options for metastatic breast cancer, also known as stage IV breast cancer that has spread to other organs such as the bones, liver, brain, and lungs, depend on various factors, including the patient’s estrogen receptor (ER) or human epidermal growth factor receptor 2 (Her2) status, gene mutations, metastatic locations, previous treatments, and involve systemic drugs and local and regional treatments, including surgery, chemotherapy, targeted biological therapy, immunotherapy, and/or radiation therapy.