Hormones, neurotransmitters, chemoattractants, and growth factors all elicit intracellular responses on binding to cell surface receptors by activating inositol phospholipid-specific phospholipase C (PLC). Activated PLC catalyzes the hydrolysis of phosphatidylinositol bisphosphate (PIP₂), a minor membrane phospholipid, to form two second messengers, diacylglycerol (DAG) and inositol (1,4,5)trisphosphate [Ins(1,4,5,)P₃]. DAG is a direct activator of protein kinase C isozymes, and Ins(1,4,5)P₃ mobilizes intracellular Ca²⁺. G protein-coupled receptors couple to the PLC-β family via G proteins, and tyrosine kinase receptors activate PLC-γ isozymes (1,2). Regardless of the PLC isozyme activated, the product is invariantly Ins(1,4,5)P₃.