ESR1 and breast cancer: Approximately 42,000 patients are projected to die from breast cancer in 2019 alone.[1] Breast cancers that do not express the estrogen receptor (ER), progesterone receptor (PR) or overexpress human epidermal growth factor receptor 2 (HER2) are commonly referred to as “triple-negative breast cancer (TNBC)” and represent approximately 11% of all breast cancers.[2] This subtype of breast cancer has a dismal prognosis, especially once the disease has metastasized.