Guanine nucleotide exchange factors (GEFs) activate GTPases by enhancing the exchange of bound GDP for GTP. Much evidence points to GEFs being critical mediators of Rho GTPase activation (Schmidt and Hall, 2002). Many GEFs are known to be highly specific for a particular GTPase, e.g. Fgd1/Cdc42 and p115RhoGEF/Rho (Hart et al., 1996, Zheng et al., 1996). Others have a broader spectrum and activate several GTPases, e.g. Vav1 for Rac, Rho, and Cdc42 (Hart et al, 1994).