Sphingomyelin (SM) is a major component of animal plasma membranes. Its production involves the transfer of phosphocholine from phosphatidylcholine and then onto ceramide. This also yields diacylglycerol as a side product. The reaction is catalysed by SM synthase, an important enzyme in the regulation of diacylglycerol and ceramide as anti- and proapoptotic stimuli. SM synthesis occurs in the lumen of the Golgi as well as on or at the cell surface. It is uncertain which SM enzyme forms are present at these differnet cellular locations. Human, mouse and Caenorhabditis elegans genomes each contain at least two different SM synthase (SMS) genes. Human SMS1 is localised to the Golgi, SMS2 resides primarily at the plasma membrane.