ESR1 and cancer: In China, breast cancer is the most prevalent cancer among urban women and the fourth leading cancer in rural areas.1 For Chinese women, the mean age at diagnosis is 45 to 55 years, which is significantly less in comparison with that in Western women.1 Epidemiological studies show that approximately 60% of Chinese women are premenopausal when diagnosed with breast cancer.2,3 Consequently, in China, selective estrogen receptor modulators (SERMs) are standard endocrine therapeutic agents.