Fig. 2. Multiple pathways controlling Cdc42 activation. In multicellular organisms, cells respond to a wide range of external signals that promote cell polarization (chemotactic signals, physical stress, cell-cell contacts). These signals are transduced by different families of receptors (purple) that can all regulate Cdc42. The signalling pathways involve several intermediates (pink) that eventually recruit and activate a Cdc42-GEF. Although some of the GEFs have been identified, the precise mechanisms involved are still to be determined.