{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"CDK4 (Cyclin-Dependent Kinase 4), a serine/threonine kinase is the catalytic subunit of a protein kinase complex that is important for cell cycle G1 phase progression. CDK4 forms an active complex with cyclin D and phosphorylates retinoblastoma protein (Rb). The phosphorylation event disrupts Rb/E2F interaction, which allows E2F to function as a transcription factor and activate genes required to guide cell progression from the G1 to S phase of the cell cycle. The activity of CDK4 is restricted to the G1-S phase, which is controlled by the regulatory subunits D-type cyclins and CDK inhibitor p16 (INK4a). Dysregulation of CDK4 activity is commonly observed in cancer, where it contributes to uncontrolled cell proliferation and tumor growth. CDK4 is frequently overexpressed or hyperactivated in various cancer types, including melanoma, glioblastoma, breast cancer (particularly in hormone receptor-positive, HER2-negative type), lung cancer, and pancreatic cancer. Alterations in CDK4 expression or activity can result from genetic mutations, gene amplification, or dysregulation of upstream signaling pathways."}]}]}