ESR1 and breast cancer: Based on the expression status of the estrogen receptor (ER), progesterone receptor, the human epidermal growth factor receptor 2 (HER2) and the proliferation marker Ki-67, breast cancer can be classified into five clinically relevant, pathological subtypes: luminal A, luminal B, triple-negative/basal-like, HER2-enriched and normal-like [2, 3].