Progression through G1 phase and transition from G1 to S phase of the cell division cycle is controlled by activation of a distinct series of serine/threonine kinase complexes which comprise of a cyclin regulatory subunit and a cyclin dependent kinase (cdk). D type cyclins are induced earlier than cyclin E in G1 and can form complexes with cdk2,-4,-5, and -6. Cyclin D1 is a nuclear protein during G1 phase and disappears from the nucleus in S-phase. Among the key cell cycle regulators, cyclin D1 has been implicated most strongly as a proto-oncogene in several human tumor types, including breast carcinomas.