P0, the major structural protein of peripheral myelin, is a homophilic adhesion molecule with a single immunoglobulin (Ig) domain, which contains a single N-linked glycosylation site and two cysteines. We have previously reported four different mutations of the myelin P0 gene in four families of Charcot-Marie-Tooth neuropathy type 1 (CMT1). In this study we found a new mutation of the myelin P0 gene in a small family of CMT1. The affected persons had an A - to - G substitution of nucleotide 245 of the myelin P0 gene in one allele, leading to a cysteine substitution for tyrosine82 in the extracellular Ig-domain. An additional cysteine in the extracellular domain may form a disulfide bond and cause an inappropriate change in the tertiary structure of the functional Ig-domain of P0.