We and others have shown that RT and CDK4/6 inhibitors can be safely combined and mediate additive-to-synergistic therapeutic effects in a variety of tumor models, including cultured human and mouse cancer cells, as well as human tumors xenografted in immunodeficient mice and mouse tumors evolving in immunocompetent syngeneic hosts [24–31]. Here, CDK4 is linked to neoplasm.