Depending on the hormone receptor expression (estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2)) and histological features, breast cancer can be divided into luminal A-like (ER+, PR+, HER2-), luminal B-like HER2- (ER+, PR+/-, HER2-), luminal B-like HER2+ (ER+, PR+/-, HER2+), HER2-enriched (ER-, PR-, HER2+), and triple-negative breast cancer (TNBC) (ER-, PR-, HER2-) (3). Here, ESR1 is linked to breast cancer.