Sphingomyelin (SPH), is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide. In humans SPH represents ~85% of all sphingolipids. In humans, sphingomyelin is believed to be the only cell membrane phospholipid not derived from glycerol. Like all sphingolipids, SPH has a ceramide core (sphingosine bonded to a fatty acid via an amide linkage). In addition it contains one polar head group, which is either phosphocholine or phosphoethanolamine.The plasma membrane of cells is highly enriched in sphingomyelin and is considered largely to be found in the exoplasmic leaflet of the cell membrane. Neutral sphingomyelinase-2 an enzyme that breaks down sphingomyelin into ceramide has been found to localise exclusively to the inner leaflet further suggesting that there may be sphingomyelin present there.