Activity of the cyclin-dependent kinases is regulated by T-loop phosphorylation, abundance of their cyclin partners and association with cdk inhibitors of the Cip/Kip or INK family of proteins. p27 Kip1 derives its ability to enforce the G1 restriction point from its inhibitory binding to cdk2/cyclin E and other cdk/cyclin complexes. In growing cells, extracellular mitogenic stimuli induces the release and degradation of p27 concomitant with a rise in cyclin D levels to effect progression through the restriction point in a pRbdependent entry into S-phase. The active complex of cyclin D/cdk4 targets the retinoblastoma protein for phosphorylation, allowing the release of E2F transcription factors that activate G1S-phase gene expression. In association with cdk2 complexes, p21 serves to inhibit kinase activity and block progression through G1/S, but it may also enhance assembly and activity in complexes of cdk4 or cdk6 and cyclin D. The carboxy-terminal region of p21 is sufficient to bind and inhibit PCNA, a subunit of DNA polymerase, and may coordinate DNA replication with cell cycle progression.