The cell cycle is regulated by the interplay of many molecules. Key among these are the cyclins which are expressed and then degraded in a concerted fashion to drive the stages of the cell cycle. Cyclins combine with cyclin dependent kinases (cdks) to form activated kinases that phosphorylate targets leading to cell cycle regulation. A breakdown in the regulation of this cycle can lead to out of control growth and contribute to tumor formation. Defects in many of the molecules that regulate the cell cycle have been implicated in cancer. Key among these are p53, the cdk inhibitors (p15, p16, p18, p19, p21, p27), and Rb, all of which act to keep the cell cycle from progressing until all repairs to damaged DNA have been completed.

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