Cyclin dependent kinases are key regulators of the progression of the cell cycle. Cdc2/p34 is a serine-threonine protein kinase of 34,000 daltons that 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. Cdc2/p34 protein must be associated with a normal cyclin protein for the M phase to be completed normally. Association with deletion mutants of cyclin halts the M phase before it is completed.