CDK4 and neoplasm: It has been verified that CDK4/6 inhibition enhances the response to PD-1 blockade in an ex vivo organotypic tumor spheroid culture system; in vivo, CDK4/6 inhibition enhances tumor regression and markedly improves overall survival rates of murine syngeneic models (Deng et al., 2018; Zhang et al., 2018).