The SH3 domain of CRK interacts constitutively with proline rich motifs present in Dedicator of cytokinesis (DOCK180), an exchange factor for RAC1. Unlike many GEFs, DOCK180 does not contain a conserved Dbl homology (DH) domain. Instead, it has a DHR2 or DOCKER domain capable of loading RAC1 with GTP (Brugnera et al 2002). Binding of DOCK180 to RAC1 alone is insufficient for GTP loading, a DOCK180-ELMO interaction is required. Engulfment and cell motility protein 1 (ELMO1) or ELMO2 form a complex with DOCK180 which functions as a bipartite GEF to optimally activate RAC1 (Gumienny et al 2001, Brugnera et al 2002, Birge et al. 2009).