ESR2 and breast cancer: Based on gene expression profiles and the status of hormone receptors, e.g. estrogen receptors alpha and beta (ERα and ERβ), progesterone receptor (PR) and overexpression of human epidermal growth factor receptor 2 (HER2), breast cancer is classified into five subtypes: i.e. luminal A(ER+ and/or PR+, HER2-, Ki-67<14), luminal B (ER+ and/or PR+, HER2-, Ki-67≥14; ER+ and/or PR+, HER2+), HER2 overexpression (ER-/PR-/HER2+), triple negative breast cancer (ER-/PR-/HER2-) (TNBC) and normal breast-like breast cancer [6].