p27^Kip1 is a cyclin-dependent kinase inhibitor involved in G1 arrest. p27^Kip1 binds to and inhibits cyclinE-Cdk2 complex, cyclinA-CDK2 and cyclinD1-CDK4 (1). p27^Kip1 is regulated by phosphorylation on serine 10 (S10) and threonine 187 (T187). Phosphorylation by CDK2 on T187 results in ubiquitination and degradation of p27^Kip1, while phosphorylation by hKIS on S10 signals nuclear export to the cytoplasm. The expression level of p27^Kip1 is high in normal cells. Downregulation of p27^Kip1 is found in many types of cancers, and decreased expression of p27^Kip1 appears to be a poor prognostic factor in several tumor models, including carcinomas of the lung, breast, colon, and prostate.