It is now clear that specific cyclin/cdk complexes
are required during specific stages of the cell cycle. Cyclin D1/cdk4,6 activity is essential for crossing the restriction point and pushing cells into replication. A major substrate
of the cyclin D1/cdk4,6 complex is the retinoblastoma (Rb) tumor suppressor protein, which when phosphorylated by this kinase complex, is inactivated. This frees the cell
from the restrictions on cell proliferation imposed by the Rb protein.