The "conventional" polyphosphoinositide pathway is important for the transmission and amplification of signals across the cell membrane. Ligand-induced activation of phospholipase C results in the hydrolysis of phosphatidylinositol 4,5-bisphosphate to produce the well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. Recently, three novel polyphosphoinositides have been implicated as important signaling molecules for cell proliferation and activation. These lipids are phosphorylated in the D-3 position of the inositol ring and appear to represent branch points from the conventional polyphosphoinositide pathway.