ELECTRONIC LETTER Position effect on PLP1 may cause a subset of Pelizaeus-

elizaeus-Merzbacher disease (PMD, MIM 312080) is an X linked disorder characterised by dysmyelination of the central nervous system (CNS)1 2 (see review by Koeppen and Robitaille3). Two main forms of the disease, a connatal and a classical type, are recognised. The connatal type has a severe course with feeding problems, progressive pyramidal and extrapyramidal symptoms, laryngeal stridor, microce-phaly, very little development, and a natural disease duration of 5–7 years. The classical type evolves into spastic tetrapar-esis, ataxia, choreoathetoid movements of the limbs, and cognitive impairment. The disorder progresses at a variable rate and some children are able to sit with support or occasionally walk, before slow regression sets in. Seizures may occur in both types (table 1). PMD has been shown to be caused by mutations in the proteolipid protein (PLP1) gene