ESR1 and breast cancer: Breast cancer, especially triple-negative breast cancer (TNBC) which do not express any of the three markers of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), is the most common cancer and the main cause of cancer-related mortality among women worldwide (Coley, 2008; Brewster et al., 2014; Palmer et al., 2014).