Themes in Signaling   Different signaling pathways are presented as distinct signaling cassettes (colored boxes). In several cases the signaling cassettes do not include all the known components of a given path way. Also shown, examples of stimulatory and inhibitory signals for the different pathways. For example, in addition to activation of the MAP kinase signaling cascade, Ras activates PI-3 kinase and Cdc42. Stimulation of PI-3 kinase leads to activation of PDK1 and PKB, two kinases that regulate various metabolic processes and prevent apoptotic death. In addition, PI-3 kinase activation stimulates generation of hydrogen peroxide which in turn oxidizes and blocks the action of an inhibitory protein tyrosine phosphatase (PTP). The signaling cassettes presented in the figure regulate the activity of multiple cytoplasmic targets. However, the Ras/MAP, STAT, JNK, and PI-3 kinase signaling pathways also regulate the activity of transcriptional factors by phosphor ylation and by other mechanisms.