ESR1 and breast cancer: Based on the expression levels of hormone receptors (estrogen receptor, progesterone receptor) and HER2/neu (human epidermal growth factor receptor 2), breast cancer can be classified into three subtypes, including hormone receptor-positive (ER+ or PR+)/HER2-negative (HER2-) subtype, hormone receptor-negative (ER- and PR-)/HER2-positive (HER2+) subtype, and hormone receptor-negative/HER2-negative (triple-negative) subtype.