Phospholipase C (PLC) isozymes are a group of related proteins that cleave the polar head group from inositol phospholipids, typically in response to signals from cell surface receptors. They hydrolyze the highly phosphorylated lipid phosphatidylinositol 4,5-bisphosphate (PIP2) generating two products: inositol 1,4,5-trisphosphate (IP3), a universal calcium-mobilizing second messenger, and diacylglycerol (DAG), an activator of protein kinase C. PLC-beta isoforms are regulated by heterotrimeric GTP-binding proteins. PLC-beta 1 and 3 are widely expressed, with the highest concentrations found in (differing) specific regions of the brain. PLC-beta 2 is expressed at highest levels in cells of hematopoeitic origin; it is involved in leukocyte signaling and host defense. PLC-beta 4 is highly concentrated in cerebellar Purkinje and granule cells, the median geniculate body, whose axons terminate in the auditory cortex, and the lateral geniculate nucleus, where most retinal axons terminate in a visuotopic representation of each half of the visual field.