<p>Ras proteins are membrane-associated molecular switches that bind GTP and GDP and slowly hydrolyze GTP to GDP [<cite idref="PUB00004087"/>]. This intrinsic GTPase activity of ras is stimulated by a family of proteins collectively known as 'GAP' or GTPase-activating proteins [<cite idref="PUB00000983"/>, <cite idref="PUB00000726"/>]. As it is the GTP bound form of ras which is active, these proteins are said to be down-regulators of ras.</p><p>The Ras GTPase-activating proteins are quite large (from 765 residues for sar1 to 3079 residues for IRA2) but share only a limited (about 250 residues) region of sequence similarity, referred to as the 'catalytic domain' or rasGAP domain.</p><p>Note: There are distinctly different GAPs for the rap and rho/rac subfamilies of ras-like proteins (reviewed in reference [<cite idref="PUB00004162"/>]) that do not share sequence similarity with ras GAPs.</p>