p27KIP1, an inhibitor of cyclin-dependent kinases.

The identification of a family of proteins that stoichiometrically regulate the activation of the G1 cyclin-dependent kinases has added to our understanding of the process of commitment to the mitotic cycle. The properties of p27 as a CDK binding protein, the ability of environmental signals to regulate the expression of p27, and the observation that p27 may link the accumulation of G1 CDK complexes with activation of the CDK2 kinase, suggest it may have a critical role in establishing a threshold for G1 cyclin/CDK accumulation prior to activation of CDK2 kinase and entry into the mitotic cycle.