CDK4 (Cyclin-Dependent Kinase 4) is an essential protein that plays a crucial role in regulating the cell cycle, particularly the transition from the G1 phase to the S phase. It works by forming a complex with Cyclin D, which activates its kinase activity, allowing the cell to progress toward DNA replication. Dysregulation of CDK4 can lead to uncontrolled cell division, contributing to the development of various cancers. As a result, CDK4 has become a target for cancer therapies, with inhibitors designed to block its activity and slow down tumor growth, highlighting its importance in cancer research and treatment.