ESR1 and breast cancer: According to molecular heterogeneity profiling, breast cancer cells can be largely characterized into five intrinsic subtypes: 1) luminal type A expressing high estrogen receptor alpha (ERα) together with progesterone receptor (PR); 2) luminal type B expressing low ERα along with high human epidermal growth factor 2 (HER2); 3) HER2-positive (HER+) possessing luminal characteristics but lack ERα; 4) basal type, also called triple-negative meaning ER-/PR-/HER2-; and 5) normal-like expressing high levels of alcohol dehydrogenase 1B (ADH1B), similar to luminal type A [2,3].