X-linked Charcot-Marie-Tooth disease is caused by mutations in the gene for the gap junction protein connexin32. This protein is expressed in peripheral nerve and present in noncompacted myelin, where it likely forms channels around and across the myelin sheath. Studies in cell culture and in transgenic mice show that connexin32 mutations can cause a loss of channel function or a gain of toxic effects on myelinating Schwann cells or both, with resulting peripheral nerve degeneration.