Human colorectal tumor cell lines were established which express wildtype p21 or p21 with a mutation at codon 46 (Cys) or 140 (Gly) on IPTG treatment (LacSwitch). The IPTG-induced wildtype p21 bound to CDK2 and PCNA and inhibited CDK activity in the cells and reduced cell growth rate; whereas, both IPTG-induced mutated p21 proteins neither bound to CDK2 nor affected the CDK activity but did bind to PCNA, and they did not affect the cell growth rate. Wildtype p21 suppressed apoptosis and enhanced survival of X-ray-irradiated or adriamycin-treated cells; but, mutated p21 neither suppressed apoptosis nor affected cell survival. When cells were treated with mimosine, a p53-independent p21-inducer, or butyrolactone I, a specific inhibitor of CDK, cellular endogenous p21 was induced and X-ray or adriamycin-induced apoptosis was blocked. These results suggest that CDK-binding or CDK-inhibitory activity of p21 is required to prevent apoptosis, i.e., CDK is required for apoptosis in human tumor cells.