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. The ataxia telangiectasia-mutated gene (ATM) encodes a protein kinase that acts as a tumor suppressor. ATM activation by ionizing radiation damage to DNA stimulates DNA repair and blocks progression through the cell cycle Cdc25 is a protein phosphatase responsible for dephosphorylating and activating cdc2, a crucial step in regulating the entry of all eukaryotic cells into the M-phase of the cell cycle. Cdc25 is phosphorylated throughout interphase but not in mitosis. The G2/M DNA damage checkpoint prevents the cell from entering mitosis (M phase) if the genome is damaged. The Cdc2-cyclin B kinase is pivotal in regulating this transition. During G2 phase, Cdc2 is maintained in an inactive state by the kinases Wee1 and Myt1. Yeast sense glucose in their environment and alter gene expression to match their nutritional needs. In a glucose-rich environment, glycolysis is activated, glucose transport is increased and gluconeogenesis repressed to use glucose to make energy. The Snf1 protein kinase is a central component of the signaling pathway for glucose repression in yeast. On removal of glucose, gene repression is relieved via a mechanism that requires the pophorylation of Mig1 protein repressor by Snf1 protein kinase complex.