Gangliosides are sialyated glycosphingolipids composed of ceramide and an oligosaccharide. These molecules are found in the plasma membrane, concentrated in lipid rafts. Gangliosides were first discovered in the brain, which is a particularly rich source of these molecules. Functionally, they appear to contribute to signal transduction. GD2, in particular, has two sialic acid (NANA) groups and is used clinically as a marker for circulating neuroblastoma cells. It is also found on melanoma cells.