CDK4 and neoplasm: Upon release from CDK4/6 inhibition, G1 synchronized cells that have not undergone irreversible cellular senescence will enter the S phase simultaneously when they are predicted to be maximally susceptible to cytotoxic drugs; or alternatively, 2) tumor cells treated first with a cytotoxic drug active at either the S or M phase will lead to arrest at either the G2/M or M checkpoint, respectively, and eventually to apoptosis.