The p27Kip1 gene codes for a cyclin-dependent kinase inhibitor implicated in G1 arrest by transforming growth factor beta, cell-cell contact, agents that elevate cyclic AMP, and the growth-inhibitory drug rapamycin. p27 binds to and inhibits complexes formed by cyclin E-cdk2, cyclin A-cdk2, and cyclin D-cdk4. The involvement of p27 in the negative regulation of cell proliferation suggests that it may also function as a tumor suppressor gene. The degradation of p27, which is triggered by its CDK dependent phosphorylation and subsequent ubiquitination by SCF complexes, is required for the cellular transition from quiescence to the proliferative state.