<p>Phosphatidylinositol 3-kinase (PI3K) (<db_xref db="EC" dbkey="2.7.1.137"/>) is an enzyme that phosphorylates phosphoinositides on the 3-hydroxyl group of the inositol ring. A subset of PI3Ks has the capacity to bind and be activated by the GTP-bound small GTPase p21Ras (Ras). PI3Ks are recognised as one of the principal effectors of Ras signalling to the cell-cycle control machinery. </p> <p> In the structure of the Ras-PI3K gamma complex, contacts between the two molecules are made primarily via the so-called switch I region of Ras and the PI3K RBD. The RBD fold comprises a five-stranded mixed beta-sheet, flanked by two alpha-helices. Interaction between Ras and the PI3K RBD is primarily polar in character and, as characterised by kinetic measurements, is reversible and transient [<cite idref="PUB00007087"/>].</p>