The best characterized receptors for mediating semaphorin signaling are members of the neuropilin and plexin families of transmembrane proteins. Neuropilins form complexes with Plexin-A which in turn can act as a signaling moiety. Also, when complexed with neuropilin-1, plexin-A1 can associate directly with the FERM domain containing guanine nucleotide exchange factor (GEF) FARP2. FARP2 exerts GEF activity for Rac but not Cdc42 and Rho.