Metastatic breast cancer patients treated with cyclin-dependent kinase 4/6 inhibitors were enrolled and retrospectively divided according to angiotensin-converting enzyme inhibitor/angiotensin II receptor blocker use: angiotensin-converting enzyme inhibitor/angiotensin II receptor blocker (+) and angiotensin-converting enzyme inhibitor/angiotensin II receptor blocker (-). The gene discussed is CDK4; the disease is breast cancer.