Phosphoinositide metabolism plays a central role in the transduction of signals triggered by a wide variety of growth factors and hormones. Phosphoinositides have been implicated in cellular processes as diverse as proliferation and differentiation, cytoskeletal rearrangement, cell survival, and vesicle trafficking (reviewed refs. 2 and 2 ). An ever-increasing number of lipid kinases are being discovered, each with its own substrate specificity, cellular localization, and unique biological function.