Cyclin D1 encodes the regulatory subunit of a holoenzyme that phosphorylates and inactivates the retinoblastomaprotein and promotes progression through the G1-S phase of the cell cycle.
The active complex of cyclin D/CDK4 targets the retinoblastomaprotein for phosphorylation, allowing the release of E2F transcription factors that activate G1/S-phase gene expression (3).