The CDK4 protein, or cyclin-dependent kinase 4, is a crucial regulator of the cell cycle, particularly in the G1 phase. It is a protein kinase that, in complex with cyclin D, phosphorylates the retinoblastoma tumor suppressor protein (pRb), leading to cell cycle progression from the G1 phase to the S phase. This process is essential for cell growth and division. CDK4 is often dysregulated in cancer, with overexpression or hyperactivity contributing to uncontrolled cell proliferation. Mutations in CDK4 can lead to its constitutive activation, which has been implicated in various types of cancer, including melanoma, glioblastoma, and breast cancer. The CDK4/6 inhibitors, such as palbociclib, ribociclib, and abemaciclib, have been developed as targeted therapies to treat certain breast cancers by inhibiting the activity of CDK4/6, thereby blocking cell cycle progression.