ESR1 and breast cancer: Joint estrogen receptor (ER) and progesterone receptor (PR), or hormone receptor (HR), status and human epidermal growth factor receptor 2 (HER2) overexpression serve as the basis of four well-established molecular subtypes of breast cancer: HR+/HER2−, HR+/HER2+, ER−/PR−/HER2−, or triple-negative (TN), and HR−/HER2+.