<p> Proteins containing a RhoGAP (Rho GTPase Activating Protein) domain usually function to catalyze the hydrolysis of GTP that is bound to Rho, Rac and/or Cdc42,inactivating these regulators of the actin cytoskeleton. The 53 known human RhoGAP domain-containing proteins are the largest known group of Rho GTPase regulators and significantly outnumber the 21 Rho GTPases they presumably regulate. This excess of GAP proteins probably indicates complex regulation of the Rho GTPases and is consistent with the existence of almost as many (48) human Dbl domain-containing Rho GEFs that act antagonistically to the RhoGAP proteins by activating the Rho GTPases. Phylogenetic analysis offers evidence for frequent domain duplication and for duplication of the entire genes containing these GAP domains [<cite idref="PUB00010623"/>].</p>