Based on both molecular and histological evidences, breast cancer could be categorized into three groups, breast cancer expressing hormone receptor: estrogen receptor (ER+) or progesterone receptor (PR+); breast cancer expressing human epidermal receptor 2 (HER2+) and triple-negative breast cancer (TNBC) (ER−, PR−, HER2−) (2). Here, ESR1 is linked to breast cancer.