Mutations in GJB1, the gene encoding the gap junction protein connexin32 (Cx32), cause X-linked Charcot-Marie-Tooth disease, an inherited demyelinating peripheral neuropathy. We generated transgenic mice that express the R142W mutation in myelinating Schwann cells. The R142W mutant protein was aberrantly localized to the Golgi, indicating that it does not traffic properly, but the molecular organization of the myelin sheath, including the localization of Cx29, another connexin expressed by myelinating Schwann cells, was not disrupted. In a wild type background, this mutation dramatically decreased the level of wild type mouse Cx32 in immunoblots of sciatic nerve and caused demyelination. The expression of wild type human Cx32 with the same transgenic construct had different effects-increased amounts of Cx32, normal localization of Cx32 at nodes and incisures, and split myelin sheaths. Thus, the R142W mutant protein has dominant effects that are distinct from overexpression.