The primary objective of this cohort study in the clinical practice setting was to investigate the potential association of clinicopathologic characteristics and treatment type with survival outcomes in a large dataset of patients with hormone receptor–positive, ERBB2–negative metastatic breast cancer undergoing disease progression with ET plus CDK4/6i treatment, with the goal of providing useful insights to support the decision-making process in this challenging clinical practice setting. The gene discussed is CDK4; the disease is breast carcinoma.