The Ras switch. Ras proteins are switches that relay signals initiated when transmembrane receptors bind ligand. Activated receptors recruit GEFs by assembly of multiprotein complexes (eg, including SOS) or more indirectly, by evoking lipid modifications that recruit GEFs, such as RasGRPs, to cytosolic membrane surfaces. GEFs promote exchange of GDP for GTP on Ras. When bound to GTP, Ras adopts a conformation in which the “switch” regions are stabilized and can interact productively with various downstream effectors. Thus, Ras connects extracellular stimuli to intracellular networks that compute and execute cell fate decisions. The Ras signal is terminated by GTP hydrolysis, which is largley dependent on GAPs, such as neurofibromin (NF1) or p120 RasGAP.