Phosphatidylinositol 3 (PI3) -kinase participates in insulin-stimulated glucose uptake, PDGF-induced membrane ruffling, and G-protein receptor signaling. It exists as a heterodimer of 85 kDa (p85) and 110 kDa (p110) subunits. The p85 subunit associates with and serves as a substrate for activated growth factor receptor tyrosine kinases. p85 regulates the p110 catalytic subunit by acting as the link between PI3-kinase and the ligand-activated receptor. Four isoforms of p110 have been identified (α, β, γ, and δ). The p110α isoform contains an N-terminal region involved in p85 binding and a C-terminal kinase domain. p85/p110α-type PI kinase phosphorylates the D-3 and D-4 position of the inositol ring of PI, thereby producing PtdIns(3)P, PtdIns(3,4)P, PtdIns(3,4,5)P, PtdIns(4)P, and PtdIns(4,5) P. During induction of chemotaxis by the chemokine SDF-1α, PI3-kinase regulates adhesion and ERM protein redistribution in the lymphocyte plasma membrane. In addition, PI3-kinases activate other signaling molecules, such as p70 S6 kinase and Akt/protein kinase B. Thus, p85/p110α-type PI kinase is a ubiquitously expressed kinase that is involved in a variety of cell signaling cascades.