Hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2−) disease is the most common breast cancer subtype, and the proportion of patients with the HR+/HER2− subtype increases with age, from 64.8% among patients aged < 50 years to 80.1% among patients aged ≥ 75 years (3). The gene discussed is ERBB2; the disease is breast cancer.