 Cyclin E/CDK2 plays a critical role in the G1 phase and in the G1-S phase transition.Cyclin E/CDK2 phosphorylates retinoblastoma protein (Rb) to promote G1 progression.Hyper-phosphorylated Rb will no longer interact with E2F transcriptional factor, thus release it to promote expression of genes that drive cells to S phase through G1 phase.