Most of Charcot-Marie-Tooth (CMT) 1 families are associated with a duplication in chromosome 17p11.2-p12, which includes the gene encoding peripheral myelin protein-22 (PMP-22). Point mutations of the Po gene have been identified in a few of the CMT 1 families in whom no duplication was found. We investigated a new mutation of the Po gene in one of those families. A to G substitution of nucleotide 389 in exon 3 resulted in Lys 131 Arg substitution. This structural change of extracellular domain of Po would alter the function of Po and result in an impairment of peripheral myelin compaction.