ESR1 and breast cancer: According to the expression level of certain biomarkers, including the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2), breast cancer is divided into five main subtypes: Luminal A (ER+, PR+), Luminal B/HER2-negative, Luminal B/HER2-positive, HER2-enriched, and triple-negative breast cancer (TNBC) (ER−, PR−, HER2−) [2,3].