ESR1 and breast cancer: Breast cancer (BC) is the most common type of cancer in women worldwide, with approximately 1,670,000 new cases globally.1 Although 90% of patients with BC survive for over 5 years, up to 10% of patients experience disease recurrence and die of disease progression after curative surgery.2,3 TNM stage, tumor grade, estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status are major predictive markers for recurrence4; they have been incorporated into management guidelines and are used to personalize treatment regimens with targeted agents.