It has been demonstrated that Ras is involved in in-. Furthermore, the activation of fos gene promoter following IL-3 stimulation was almost completely abolished when Ras(S17N) was induced. Under these conditions, Ras(S17N) exhibited no inhibitory effect on IL-3-dependent proliferation assessed by the increase of cell numbers and a mitochondrial enzyme activity. The results indicate that Ras-dependent pathways, including the Raf/MAPK/Fos pathway, are dispensable for IL-3-induced growth stimulation. When BaF3 cells were treated with a tyrosine kinase inhibitor, herbimycin A, IL-3-dependent proliferation of the cells was impaired, suggesting that tyrosine kinase-mediated pathways are critical for growth promotion. On the other hand, apoptotic cell death caused by deprivation of IL-3 was prevented by the induction of the activated mutant Ras(G12V), although the rate of cell number increase was markedly reduced. Thus, it is likely that Rasindependent pathways play important roles to facilitate the proliferation although they may not be essential for IL-3-stimulated antiapoptotic signal transduction.In various types of cells, Ras functions as a molecular switch that regulates intracellular signaling pathways for proliferation, differentiation, and other physiological responses. Tyrosine kinase receptors, such as epidermal growth factor receptor and platelet-derived growth factor receptor, when stimulated by their specific ligands, form a complex with a variety of signal-transducing molecules including phosphatidylinositol 3-kinase, Ras-GTPase activating protein (Ras-GAP), 1 phospholipase C-␥1, and adaptor proteins (e.g. Grb-2, Nck, and Shc) through the interaction between specific phosphotyrosines on the receptor and Src homology 2 (SH2) domains of the binding molecules Schlessinger, 1993). Among the above components of the signal-transducing complex, Shc and Grb-2 are well characterized as elements that link the receptor and a Ras-guanine nucleotide exchange factors (Ras-GEFs), such as mSos-1. Adaptors and Ras-GEFs are known to participate also in Ras regulation through non-tyrosine kinase-type receptors including interleukin 2 (IL-2), IL-3, and T cell antigen receptors (Burns et al., 1993;Cutler et al., 1993;Ravichandran et al., 1993;Sato et al., 1993;Buday et al., 1994;Reif et al., 1994;Ravichandran and Burakoff, 1994;Welham et al., 1994). The active GTP-bound form of Ras specifically binds RasGAPs (Boguski and McCormick, 1993), c-Raf-1 (Avruch et al., 1994), B-Raf (Moodie et al., 1994;Vaillancourt et al., 1994), phosphatidylinositol 3-kinase (Rodriguez-Viciana et al., 1994), Ral-guanine nucleotide dissociation stimulator (Hofer et al., 1994;Kikuchi et al., 1994;Spaargaren and Bischoff, 1994), mitogen-activated protein kinase (MAPK)/extracellular signalregulated kinase (ERK) kinase (MEK) kinase (MEKK) (LangeCarter and Johnson, 1994), and Rin1 (Han and Colicelli, 1995), among which Raf proteins are best characterized as direct targets of Ras (Daum et al., 1994;Marshall, 1995). After the binding of Ras, the serine/t...