Macrophages lacking all the three isoforms of VAV did not affect FCGR-mediated phagocytosis suggesting that RAC1 is regulated by GEFs other than VAV downstream of the FCGR (Hall et al 2006). DOCK180, a member of GEFs, is found to be involved in the activation of RAC1. DOCK180 associates with the adaptor protein CRKII and the complex is found to accumulate at the phagocytic cup. DOCK180 is recruited to the sites of phagocytosis by binding to SH3 domain of CRKII through its proline-rich motif (Hasegawa et al 1996). CRKII is likely recruited to the activated FCGR complex by binding phosphorylated ITAM tyrosines on the receptor or through other phosphotyrosines on ancillary proteins that are recruited to the receptor complex (Lee et al 2007). Unlike the usual GEFs, DOCK180 does not contain the conserved Dbl homology (DH) domain. Instead, it has a DHR-2 or DOCKER domain capable of loading RAC with GTP (Brugnera et al 2002). Binding of DOCK180 to RAC alone is insufficient for GTP loading, and a DOCK180-ELMO interaction is required. ELMO1, as well as ELMO2, form a complex with DOCK180 and they function together as a bipartite GEF to optimally activate RAC (Gumienny et al 2001, Brugnera et al 2002).