Proteolipid protein (PLP) is the major myelin protein of the CNS and is believed to have a structural role in maintaining the intraperiod line of compact myelin. An isoform, DM-20, produced by alternative splicing of exon 3B is expressed earlier than PLP in the CNS and may be involved in glial cell development. DM-20 is also present in myelin-forming and non-myelin-forming Schwann cells, olfactory nerve ensheathing cells, some glial cell lines and cardiac myocytes. Molecular studies suggest the existence of a PLP gene family with sequence similarities between molecules of different species. Such studies also lend credence to the suggestion that PLP and/or DM-20 may function as a membrane pore. Mutations in the PLP gene occur in several animal species and cause severe pleiotropic effects on myelination. In man this presents as Pelizaeus-Merzbacher disease (PMD). The phenotype of such mutants is characterized by dysmyelination with myelin of abnormal periodicity, paucity of mature oligodendrocytes and astrocytosis. Duplication of the PLP gene in transgenic animals or in one form of PMD also results in dysmyelination. X-linked spastic paraplegia (SPG2) is allelic to PMD and is associated with PLP mutations in which the levels of the DM-20 isoform are probably relatively normal. The effects of PLP gene dosage on CNS myelination can be compared in many ways to the variety of phenotypes in the PNS in hereditary neuropathies of the Charcot-Marie-Tooth type in which the peripheral myelin-22 gene is mutated.