Distribution of breast cancer subtypes by age groups. Percentages of breast cancer patients with HR+/HER2−, HR+/HER2+, HR−/HER2+, and HR−/HER2− status in the SEER-White, SEER-Asian, and ABCCG-Asian cohorts are shown across three age groups: young (<40 years), alleged premenopausal mid-age (40-49 years), and alleged postmenopausal age (≥50 years). ABCCG, Asian Breast Cancer Cooperative Group; HER2, human epidermal growth factor receptor 2; HR, hormone receptor; SEER, Surveillance, Epidemiology, and End Results Program.