PI3 Kinase (Phosphatidylinositol 3-Kinase) is responsible for phosphorylation of the 3 position of the inositol ring of PI(4,5)P2, to generate PI(3,4,5)P3, a potent second messenger required for survival signaling, and insulin action. PI3 Kinase is a heterodimeric complex composed of an 85 kDa regulatory subunit and a 110 kDa catalytic subunit. Tyrosine phosphorylation of growth factor receptors creates docking sites for binding of p85 (through its SH2 domains) on the receptors; p85 brings with it p110, which is then proximal to its phospho-lipid substrate on the membrane. PI3 Kinase is also activated by Ras, and by the β,γ subunits of heterotrimeric G-proteins. PI3 Kinase is inhibitable by wortmannin, a useful tool for the study of the PI3 Kinase signaling pathway.