The Cdc2 p34-cyclin B complex plays a critical role in the cell cycle by regulating the G2 to M transition. Also referred to as M-phase promoting factor or MPF, this complex is a required component of the cell cycle machinery and is necessary for cell entry into mitosis. The Cdc28 protein represents the S. cerevisiae counterpart of human Cdc2 p34 and has been found complexed to a regulatory protein, termed p13suc 1, in addition to cyclin B. The human homolog of p13suc 1 has been identified and designated Cks1 p9. Null mutations in the p13suc 1 and Cks1 p9 genes result in the arrest of the cell cycle at either the G1 or G2 phase, suggesting that the proteins may also regulate the activity of cyclin dependent kinases that act at critical points early in the cell cycle.