Inhibitory molecules in CNS myelin affect axonal regeneration after injury. In the past year, myelin-associated glycoprotein (MAG), a well-characterized myelin protein, has been identified as an inhibitor of axonal regeneration. This finding, together with its established ability to promote outgrowth, defines MAG as a bifunctional molecule. MAG has also been included in a family of sialic acid binding proteins, providing a clue to the identity of the MAG receptor. MAG knockout mice reveal that MAG is not essential for the initiation of myelination; however, it plays an important role in maintaining a stable interaction between axons and myelin.