ESR1 and breast cancer: Based on the expression of molecular markers, estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), BC can be categorized into three subtypes: luminal-like (hormone receptor (HR)+/HER2-), HER2 positive (HER2+), and triple-negative (HR-, HER2-) breast cancer (Yeo and Guan, 2017; Barzaman et al., 2020).