Peripheral myelin protein 22 (PMP22), a membrane glycoprotein, plays a significant role in the formation and/or maintenance of compact myelin in the peripheral nervous system. We studied two pedigrees with Dejerine-Sottas disease and identified two novel mutations in the PMP22 gene: one a 2-bp deletional mutation at nucleotide positions 426 and 427 of exon 4 (this is predicted to alter the reading frame at leucine 80 and thus to lead to frame-shifted translation), and the other a guanine to thymine substitution at nucleotide position 636 leading to a cysteine substitution for glycine 150. Both mutations were located in the putative transmembrane domains reported in many cases of Charcot-Marie-Tooth neuropathy, Dejerine-Sottas disease, and hereditary neuropathy with liability to pressure palsies. The results suggest an important role for the putative transmembrane domains of PMP22 in its function.