The regulation of cell proliferation in multicellular organisms is a complex process, which is primarily regu-lated by external growth factors provided by surrounding cells. Once induced to proliferate, the passage through the mitotic cell cycle is directed by the components of the so called cell cycle machinery. Over the last years research on growth factor signaltransduction and the components of the cell cycle system has lead to a detailed knowledge of the mechanisms by which growth factors transmit their signals and of the relationship between the components of the cell cycle machinery. The remaining question how the growth factor mediated signal-transduction cascades couple with the cell cycle regulators has recently been a focus of interest.Keywords: Ras; Raf; cell cycleThe Ras GTP-binding protein and Raf kinases have a central role in transmitting growth factor-triggered signals. The Ras protein is an important e ector of growth factor receptors and activates di erent signaltransduction cascades (Daum et al., 1994;Avruch et al., 1994;Rodriguez Viciana et al., 1996;Symons, 1996). One of these is the Ras/Raf/Mek/Erk cascade (Daum et al., 1994). By direct interaction the Ras protein targets the Raf kinase to the plasma membrane where the catalytic activity of the kinase becomes activated. Subsequently Raf stimulates the Mek kinase by phosphorylation, which then phosphorylates and activates the Erk kinases. The Erk kinases shuttle into the nucleus and induce the activity of transcription factors. By employing constitutively active and dominant negative mutants of the Ras and Raf proteins it has been shown that the Ras/Raf signaltransduction cascade plays a crucial role in the regulation of cell proliferation by growth factors (Mulcahy et al