ESR1 and breast cancer: Based on expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), breast cancer can be categorized into three subtypes: hormone receptor-positive (ER+ or PR+), HER2-positive (ER-, PR-, and HER2+), and triple-negative breast cancer (TNBC) (ER-, PR-, and HER-) types [2, 3].