ESR1 and breast cancer: ER and HER2 are important treatment targets in breast cancer.7,9 Based onimmunohistochemical classification, the luminal B subtype is divided into luminal B (HER2–) andluminal B (HER2+) subtypes according to HER2 status, each of which is associated with differenttherapeutic strategies.7,9 Anti-HER2 therapy is the standard of care for patients with luminal B(HER2+) tumors but not luminal B (HER2–) tumors.7,9