A reduced expression of cyclin kinase inhibitor p21 WAF/CIP1 is regulated by the ubiquitin-proteasome protein degradation system in HeLa and Saos-2 cells when exposed to a proteasome inhibitor,MG-132.Skp2 is a member of the F-box family of substrate-regulation subunits of SCF ubiquitin-protein ligase complexes. To investigate the relationship between the high expression of Skp2 and the low expression of p21, an experiment for inhibiting the expression of Skp2 by antisense oligonucleotide was performed. The high Skp2 expression was confirmed to correlate with the low expression of p21. Furthermore,a similar correlation was observed in the over-expression of Skp2 or a mutant Skp2 lacking the F-box in HeLa cells. By performing immunofluorescence staining assays,Skp2 was found to be co-localized with p21 in nucleus of HeLa cells. GST pull-down and co-immunoprecipitation assays revealed that Skp2 bound to p21 directly,and the domain of interaction was not in the F-box construct of Skp2. These results provided evidence of a role for Skp2 in regulating p21 stability,in which F-box domain of Skp2 played an important role.