The cell division in Eukaryota is regulated by a protein complex consisting of p34cdc2 protein kinase (or related CDK) and cyclin. The p34cdc2 protein kinase plays a catalytic role, whereas the cyclin moiety is a regulatory subunit. The p34cdc2/cyclin complex is required at two control points of the cell cycle: between G1 and S phase and between G2 and mitosis. This paper presents characterization of the key molecules regulating progression throughout the cell division cycle.