The ADP-ribosylation factor (Arf) family comprises a group of structurally and functionally conserved 21 kDa proteins, which are members of the Ras superfamily of regulatory GTP-binding proteins. Arf is involved in intracellular protein traffic to and within the Golgi complex. Arf has a number of disparate activities including maintenance of organelle integrity, assembly of coat proteins, as a co-factor for cholera toxin and as an activator of phospholipase D. Like other small GTPases, Arf is found to be active when bound to GTP and inactive when bound to GDP. Arf’s activation is dependent upon guanine nucleotide exchange factors (GEFs) which increase the rate of exchange of bound GDP with GTP. All GEFs have a highly conserved Sec7 domain. GEF activity lies in the Sec7 domain and this activity has been shown to be inhibited by the fungal metabolite brefeldin-A (BFA). A small group of GEFs which are insensitive to brefeldin-A (BFA) include cytohesin-1 (B2-1), cytohesin-2 (ARNO), cytohesin-3 (ARNO3), and cytohesin-4. All cytohesins function in the cell periphery and contain a pleckstrin homology (PH) domain. The PH domain has been shown to interact with phosphatidylinositol 3,4,5-triphosphate and is believed to promote membrane targeting of the cytohesins. Recruitment of the cytohesins to the membranes can occur in response to tyrosine kinase receptor activation. This response appears to require the activation of phosphatidylinositol 3-kinase (PI 3-kinase).