CCNA2 and neoplasm: Notably, transition from G1 to S phase is mediated by CDK4-cyclinD and CDK6-cyclinD, together with CDK2-cyclinE, all of which can phosphorylate and inactivate the retinoblastoma (Rb) protein, an anti-tumor and anti-proliferative protein that limits the transcription of E2F target genes, such as CCNA2 (CyclinA2) and CDK1. CDK1-cyclinA and CDK2-cyclinA drive cells through the S phase, whereas mitosis depends on CDK1-cyclinB7,8.