Ras protein is a GTP-binding protein, and acts as a signal transducer in fibroblast, lymphoid, myeloid, and neuronal cells. In all cases, tyrosine kinases, intrinsic to or associated with receptors, seem to play an important role for the activation of Ras in response to extracellular stimulations. A GDP/GTP exchange regulator and a GTPase stimulatory protein are thought to mediate signals from the kinases. The active Ras.GTP can cause different phenotypes, that is, proliferation, transformation, activation, or differentiation, depending on cell types, although it is not yet clear what is the primary target of the active Ras-GTP or how the various phenotypes are determined downstream of Ras protein.