ESR1 and breast cancer: Breast cancer remains the most common cancer diagnosed among women in the United States and is the second leading cause of cancer-related deaths, with approximately 41,000 patients projected to die from this disease in 2018 alone.[1] The prognosis for patients with metastatic breast cancer varies based on many factors including estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status.