Patients diagnosed with HR + /HER2-ABC who were treated with CDK4/6i in clinical routine between November 2016 and December 2020 at four certified German Breast Cancer Centers (Saarland University Medical Center, University Medical Center Charité Berlin, University Medical Center Bonn and University Medical Center Schleswig–Holstein, Campus Kiel) were retrospectively identified. This evidence concerns the gene CDK4 and breast cancer.