Guanine nucleotide exchange factor (GEF) for Rho and Rac. GEF proteins activate small GTPases by exchanging bound GDP for free GTP (By similarity). Along with DOCK1, mediates CRK/CRKL regulation of epithelial and endothelial cell spreading and migration on type IV collagen (PubMed:19004829). Interacts with CRK and CRKL. Associated with the edge of the plasma membrane in Caco-2 intestinal epithelial cells spreading on type IV collagen. The DOCKER domain may mediate some GEF activity. Belongs to the DOCK family.