ESR1 and breast cancer: Gallen expert consensus, i.e., luminal breast cancer expressing ERα and/or the progesterone receptor (PR), overexpressing (luminal B)—or not (luminal A/B)—the human epidermal growth factor receptor 2 (HER2) and/or having (luminal B)—or not (luminal A)—a high proliferative index (Ki-67) [1,2,3].