Based on these findings, breast cancers are classified as luminal subtype A (estrogen receptor (ER)-positive and/or progesterone receptor (PR)-positive, human epidermal growth factor receptor 2 (HER2)-negative and low Ki67), luminal subtype B (ER-positive, PR-positive or negative, HER2-positive and has high Ki-67), HER2-positive (HER2 overexpressing), basal-like/triple-negative (ER-negative, PR-negative and HER2-negative), normal breast-like and claudin-low [16,19]. Here, ESR1 is linked to breast cancer.