ESR1 and breast cancer: BC is classified as hormone receptor (HR)-positive (HR+/HER2−, or ER+/PR+/HER2−), triple positive (HR+/HER2+, or ER+/PR+/HER2+), HER2-positive (ER−/PR−/HER2+, or HER2+), HER2-negative (ER+/PR+/HER2−, or HER2−), or triple-negative (TNBC) (ER−/PR−/HER2−).