Abstract: Methods and compositions are described that affect the GTPase activity of members of the Ras superfamily, preferably Rac, such compositions include guanine nucleotide exchange factors that modulate the GTPase activity, preferably in the presence of GEF enhancers, exemplary guanine nucleotide exchange factors being Rac-GEF and Tiam-1, which are encoded by certain nucleic acid sequences that are herein described, along with uses for the guanine nucleotide exchange factors and the nucleic acid sequences including screening for ligands which recognize Rac-GEF, regulators of Rac-GEF activity, and methods of treating pathological conditions associated or related to a Ras superfamily GTPase, including Rac.