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Kip1 is a cyclin-dependent kinase (cdk) inhibitor that was identified as a result of its role in TGFβ-induced G1 phase arrest and cell-cell contact. In vitro , p27 [Kip1] binds tightly to Cyclin D-Cdk4, Cyclin E-Cdk2, and Cyclin A-Cdk2 complexes and inhibits their activity. In normal cells, Kip1 is sequestered and its activity gradually decreases as cells reach S phase. The addition of TGF-β in early G1 blocks this decline in activity, by preventing the sequestration of Kip1. TGF-β also reduces the levels of Cdk4. Kip1 preferentially binds Cyclin D-Cdk4, but the lower levels of Cdk4 in TGF-β treated cells allow Kip1 to be available for binding to Cyclin E-Cdk2 and Cyclin A-Cdk2. Kip1 is structurally related to Cip1/WAF1, having a similar 60 amino acid sequence in the N-terminal region. A 52 amino acid Kip1 peptide (residues 28-79) from this region is sufficient to inhibit cdk activity in vitro.Immunofluorescence, Immunohistochemistry, Immunoprecipitation, Western Blotting