CDK4 and cancer: Cyclin D1 regulates the G1-S phase transition through its interaction with CDK4/6, forming the cyclin D1-CDK4/6 complex that phosphorylates the retinoblastoma protein (pRb). Hyperphosphorylated pRb releases E2F transcription factors, promoting cell cycle progression and uncontrolled proliferation in cancers, including OSCC.