The proliferation of eukaryotic cells is controlled at specific points in the cell cycle, particularly at the G1 to S and the G2 to M transitions. It is well established that the Cdc2 p34-cyclin B protein kinase plays a critical role in the G2 to M transition while cyclin A associates with Cdk2 p33 and functions in S phase. Considerable effort directed towards the identi- fication of G1 cyclins has led to the isolation of cyclin D, cyclin C and cyclin E. Cyclin D corresponds to a putative human oncogene designated PRAD1 which maps at the site of the BCL1 rearrangement in certain lymphomas and leukemias. Cyclin C complexes with the cyclin dependent kinase Cdk8. The cyclin C/Cdk8 complex has been shown to have kinase activity toward the carboxy terminal domain of RNA polymerase II. Two complexes have been identified which contain cyclin C/Cdk8.
