ESR1 and breast cancer: Generally breast cancers could be classified according to the TNM staging system[2] and their molecular groups[3], which include luminal A type (estrogen receptor (ER) + and/or progesterone receptor (PR) +, human epidermal growth factor receptor-2 (HER2)-), luminal B type (ER + and/or PR+, HER2+), HER2 over-expressing type (ER-, PR-, and HER2+), basal-like type (ER-, PR-, HER2-, cytokeratin 5/6+, and/or epidermal growth factor receptor (EGFR)+).