Currently, several types of targeted drugs, such as cyclin-dependent kinase 4/6 (CDK4/6) inhibitors (CDK4/6i), poly-(ADP)-ribose polymerase (PARP) inhibitors (PARPi), PI3K inhibitors, and AKT inhibitors, are approved for treating certain subtypes of breast cancer and ovarian cancer, but none of these drugs alone showed satisfactory clinical benefits [3,4], suggesting an urgent need and immediate significance for discovery of new biomarkers of responses, and new strategies of combinatorial treatments. This evidence concerns the gene CDK4 and breast cancer.