The Ras superfamily of small GTPases are GDP/GTP-regulated molecular switches. GDP/GTP cycling is controlled by two main classes of regulatory protein: Guanine Nucleotide Exchange Factors (GEF) promote formation of the active GTP-bound form and GTPase Activating Proteins (GAP) promote the hydrolysis of GTP and the inactive GDP-bound form. They share a common domain, the G domain, with heterotrimeric G-protein α subunits and other GTPases. Many members of this family are associated with membranes, some via post-translational modification with lipids. (Adapted from PMID:15731001).