There are two key types of regulatory molecules that control a cell's progress through the cell cycle, (1) the cyclin-dependent kinases and (2) several different cyclins (38). The cyclin-dependent kinases (CDKs) are a family of protein kinases that regulate the activity of cyclins through phosphorylation, a common biochemical reaction that activates or inactivates target proteins. CDKs are constitutively expressed in cells however their kinase activity is turned on at different stages of the cell cycle when they complex or dimerize with cyclins, forming an activated heterodimer. The activated CDK can then phosphorylate a target protein, which in turn regulates entry into the next phase of the cell cycle. In the absence of the correct cyclin,
