ESR1 and breast cancer: Nearly 70% of breast cancer is estrogen receptor-positive (ER+), human epidermal growth factor receptor 2-negative (HER2-).1 Although effective therapy for early breast cancer has reduced the 5-year recurrence rates and mortality rates, about 20% patients still experience disease relapse or metastasis.2 For patients with ER+/HER2- metastatic breast cancer (MBC), endocrine therapy and chemotherapy are both important treatment options.