MAG (Myelin-Associated Glycoprotein), also known as Siglec-4a, is a transmembrane lectin that preferentially binds to alpha-2,3-linked sialic acid terminal carbohydrates on cell surface molecules. It is expressed by myelinating oligodendrocytes and Schwann cells and plays an important role in axon-myelin stability. MAG also regulates axon structure at nodes of Ranvier, inhibits neurite outgrowth, and protects neurons from excitotoxicity. It interacts with the gangliosides GD1a and GT1b, the Nogo receptors NgR1 and NgR2/NgRH1, Integrin beta 1/CD29, and PIR-B. Proteolytic cleavage releases a soluble form that is released from myelin in large quantities.