ESR1 and breast cancer: Gallen International Expert Consensus, breast cancer is categorized into five subtypes: luminal A, luminal B human epidermal growth factor receptor-2 (HER-2)-positive, luminal B HER-2-negative, HER-2-enriched, and triple-negative based on the expression status of the estrogen receptor (ER), progesterone receptor (PR), HER-2-positive, and Ki-67 index (2).