ESR1 and breast cancer: Approximately 70% of breast cancer patients are estrogen receptor-positive or progesterone receptor-positive, commonly treated with endocrine therapy (Wolff et al., 2007); patients with overexpression of human epidermal growth factor receptor 2 (HER2) account for 15% to 20%, commonly treated with targeted therapy (Romond et al., 2005; Slamon et al., 2001); triple-negative breast cancer still lacks targeted therapy methods, and traditional chemotherapy remains the main treatment option (Duffy, McGowan & Crown, 2012).