We identified a de novo mutation in the peripheral myelin protein (PMP22) gene of a patient with Déjérine-Sottas neuropathy. Single-stranded conformation analysis of PCR-amplified DNA fragments showed an additional fragment for exon 1 in the patient, which was absent in the unaffected parents. Sequence analysis showed a de novo point mutation C85-->A that results in an amino acid substitution His12Gln in the first transmembrane domain of PMP22. This provides further evidence that sporadic cases of Déjérine-Sottas neuropathy can be due to dominant single base substitutions.