A large number of low molecular weight GTP-binding proteins of the Ras superfamily have been identified in eukaryotic cells; they regulate many fundamental processes such as cell growth, vesicle traffic and cytoskeletal organization. GTPase-activating proteins accelerate the intrinsic rate of GTP hydrolysis of Ras-related proteins, resulting in downregulation of their active form. Rac 1 is activated in a type I interferon (IFN) dependent manner; its function is required for downstream engagement of the p38 MAP kinase pathway. The p38 MAP kinase plays an essential role in IFN-dependent transcriptional regulation. The serine/threonine kinase Akt, of the phospho-inositide 3-kinase signal transduction pathway phosphorylates serine 71 of Rac 1. The superfamily of GTP-binding proteins, for which the Ras proteins are prototypes, has been implicated in regulation of diverse biological activities involving various aspects of cell growth and division.One mammalian member of the family, Cdc42, has an amino acid sequence that is similar to those of various members of the Ras superfamily proteins, including N-, K- and H-Ras, Rho proteins and the Rac proteins. On the basis of in vitro phosphorylation studies, it has been suggested that human Cdc42 may function in the signaling pathway of the EGF receptor or related growth factor receptor protein kinases. The Dbl oncogene has been shown to specifically catalyze dissociation of GDP from human Cdc42.