 The Cyclin E/CDK2 complex phosphorylates p27Kip1 (an inhibitor of Cyclin D), tagging it for degradation, thus promoting expression of Cyclin A, allowing progression to S phase.Like all cyclin family members, cyclin E forms a complex with cyclin-dependent kinase (CDK2).Cyclin E/CDK2 regulates multiple cellular processes by phosphorylating numerous downstream proteins.