The myelin brake: when enough is enough.

Myelination by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system is tightly regulated by interactions with axons. Various investigations have shed light on the signaling pathways that mediate the production of myelin, but an important question remains; that is, which signals determine when the cell stops myelinating. New studies demonstrate that in Schwann cells, this is controlled by the abundance of Dlg1, which acts to stop active myelination.