The family of small GTPase proteins is comprised of Cdc42, Rac, and Rho. Proteins of this family function as molecular switches that regulate a multitude of biological processes including cell proliferation, apoptosis, differentiation, migration, cytoskeletal reorganization, and membrane trafficking. GTPases cycle between an inactive GDP-bound conformation and an active GTP-bound conformation. Cycling between the two conformations enables these proteins to act as binary switches. The tightly regulated GTP-binding/GTPase cycle requires the coordinated action of three regulatory proteins: 1) Guanine nucleotide exchance factors (GEF), which stimulate the GTP-GDP exchange reaction, 2) GTPase activating proteins (GAP), which stimulate the GTP-hydrolytic reaction, and 3) GDP-dissociation inhibitors (GDI), which antagonize the actions of both GEFs and GAPs...