ESR1 and breast cancer: Among different breast cancer subtypes, basal-like breast cancer is often clinically defined as triple-negative breast cancer (TNBC), which lacks immunohistochemistry (IHC) expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (her2/neu) according to College of American Pathologist (CAP)/ASCO guidelines2.