The most commonly used subtypes of breast cancer are luminal A (estrogen receptor positive (ER+), progesteron receptor positive (PR+) and human epidermal growth factor receptor negative (HER2−)), luminal B (ER+, PR+/−, and HER2−), HER2-positive (ER−, PR−, and HER2), and triple-negative breast cancer (ER−, PR−, and HER2−). Here, ESR1 is linked to breast cancer.