A simplified categorization, based on the expression level of the estrogen receptor (ER), the progesterone receptor (PR), and the human epidermal growth factor 2 (HER2), has been adopted in clinical practice, and this has allowed breast cancers to be grouped into three distinct subtypes: luminal-like tumors (ER-positive and/or PR-positive and HER2-negative), HER2-positive tumors (HER2-positive, any ER, and PR-positive), and triple-negative (ER-negative, PR-negative, and HER2-negative) tumors. Here, ESR1 is linked to breast cancer.