ESR1 and breast cancer: It is well established that BC can be classified based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) into clinical subtypes including hormone receptor positive, HER2-negative (luminal A or luminal B on molecular testing), HER2-overexpressing (HER2+), or BC negative for ER, PR, and HER2 (ER-/PR-/HER2-), often referred to as triple-negative breast cancer (TNBC), most consistent with the basal subtype on molecular testing.