Cyclin-dependent kinase 4–cyclin D1 complex (CDK4–cyclin D1)

The **cyclin-dependent kinase 4–cyclin D1 complex (CDK4–cyclin D1)** is a critical serine/threonine kinase complex that regulates cell cycle progression at the G1 to S phase transition. The complex forms when cyclin D1 binds to CDK4, enabling kinase activity that phosphorylates the retinoblastoma protein (Rb), thus releasing E2F transcription factors required for DNA synthesis and further cell cycle progression. Dysregulation of this complex, typically through overexpression or gene amplification of cyclin D1 or CDK4, is a common feature in multiple cancer types, driving uncontrolled proliferation[1][4][6][7]. Selective inhibitors of CDK4/6 (such as palbociclib, ribociclib, and abemaciclib) are approved and widely used in the treatment of hormone receptor-positive breast cancer and are under evaluation for other malignancies[4][5]. The specificity of CDK4/6 inhibitors for the active conformation of the CDK4–cyclin D complex underlies their therapeutic benefit. The activity and assembly of the complex are tightly regulated by molecular chaperones (e.g., Hsp90, Cdc37), and by binding partners such as the Cip/Kip family (e.g., p21, p27), which can act as both activators and inhibitors depending on phosphorylation status and cellular context[1][2][3]. Key biomarkers for drug responsiveness include cyclin D1 and CDK4 amplification, as well as RB1 integrity. Major safety concerns related to CDK4/6 inhibition include myelosuppression and the potential for resistance mechanisms.

Other names

CDK4/cyclin D1Cyclin D1–CDK4 complexCyclin D–CDK4Cyclin D1–cyclin-dependent kinase 4 complexCyclin D/CDK4 complex

ATP-competitive inhibition of kinase activity\nBlockade of phosphorylation of RB protein, leading to cell cycle arrest at the G1 phase\nDisruption of cyclin D1–CDK4 complex activity