Cyclin dependent kinases are key regulators of the progression of the cell cycle. Early in the cell cycle Cdk2, Cdk4, Cdk6 and their associated cyclins regulate the G1 to S phase transition (1, 2). Cdk2 plays a key role in the G1/S and S/G2 transitions through its associations with cyclin D1, cyclin D2, cyclin D3, cyclin E and cyclin A. Cdk4 also forms complexes with the D type cyclins, and is thought to regulate cell growth through the G1 phase of the cell cycle (3-6). The late stages of the cell cycle are regulated by another cyclin dependent kinase, Cdc2 p34. This kinase exists as a complex with both cyclin A and cyclin B. The best characterized of these associations is the Cdc2 p34-cyclin B complex that is required for the G2 to M phase transition (7,8).