There are two points (brake-points) through which the cell must pass before it can enter cell division. Progress through each brake-point requires the presence of an active cyclin-dependent kinase (Cdk). There are specific cyclins to activate the Cdk's at different parts of the cell cycle. Activation of the cyclin-Cdk complex is tightly regulated by the phosphorylation state of the Cdk. Exogenous growth stimulators (hormones, growth factors, and cytokines) all work through an intracellular kinase cascade that drives the production and activation of early nuclear proteins that, in turn, induce transcription of the genes for cyclins, Cdk's, and other cell cycle regulators. Retinoblastoma protein regulates cell division by inactivating specific growth-promoting proteins. Therefore, mutation of the Rb gene can lead to uncontrolled cell division and thus promotion of transformed cells. p53 protein will prevent replication of cells with damaged DNA. Hence, transformed cells can only readily progress to tumors if the p53 gene is mutated in a manner that inactivates the protein product. Members of the bcl-2 family act, in homodimers and heterodimers, to shunt cells either into cell division or into apoptosis. Understanding the mechanisms by which the balance of cell cycle: apoptosis can be manipulated will lead to new ways of controlling abnormal cellular growth. Most aspects of cellular function reflect changes in phosphorylation of critical serine, threonine, and tyrosine residues on the relevant regulatory proteins. The kinases the phosphatases involved are themselves under tight control.