In this cohort study of 144 103 US patients with hormone receptor–positive/human epidermal growth factor receptor-2–negative early breast cancer who did not undergo chemotherapy between 2004 and 2014, delays in the initiation of AHT more than 150 days were associated with diminished survival compared with patients who initiated therapy before 150 days. The gene discussed is ERBB2; the disease is breast carcinoma.