ESR1 and breast cancer: Tumors expressing ER and/or PR are categorized as luminal BC, which may or may not express HER2, defined as luminal A-like (strongly ER+ and PR+; HER2−), luminal B-like/HER2− (with ER and PR levels lower than that of luminal A, but a high proliferation index) and luminal B-like/HER2+ (ER and PR levels lower than luminal A; HER+).