GM2-ganglioside (II3NeuAcGgOse3Cer) is a minor component of adult nervous tissue, but is probably an oncofetal antigen. Its biological role is unknown, but several lines of evidence indicate its potential role in cell adhesion both in the retina and in oligodendrocytes. The biosynthesis of GM2-ganglioside appears to be tightly regulated, since it is a key intermediate in complex ganglioside synthesis. The specific GM3: hexosaminyl-transferase is activated under conditions which activate cyclic AMP-dependent protein kinase, and cell transformation with retroviruses inactivates it. Catabolism of GM2 requires the concerted action of three gene products (alpha-chain, beta-chain and activator protein in a thermolabile alpha beta 2 AP complex referred to as HexA). Defects in either three components results in the neuronal storage of GM2 ganglioside and the manifestations of Tay-Sachs Disease in children or motor neuron disease in adults.