ESR1 and breast cancer: Patients with HR+/HER2− luminal BC (n = 3166, 2005–2013 at Chang Gung Memorial Hospital) were classified into two groups based on pathological and IHC features: luminal A (ER and/or PR+, Ki67 ≤ 20%, or grade 1) and luminal B (ER and/or PR+, Ki67 > 20%, or grade 2 or 3).