For example, four subtypes of breast cancer categorized by receptor status are i) luminal A (estrogen receptor (ER) positive, progesterone receptor (PR) positive, and human epidermal receptor 2 (HER2) negative), ii) luminal B (ER-positive and/or PR-positive, and HER2-positive), iii) HER2-enriched (ER-negative, PR-negative, and HER2-positive), and iv) triple-negative (ER-negative, PR-negative, and HER2 negative) breast cancer [2]. Here, ESR1 is linked to breast cancer.