A Charcot-Marie-Tooth disease 1B (CMT1B) family with a mutation of the Po gene is presented. A to G substitution of nucleotide 389 in exon 3 resulted in Lys 131 Arg substitution. Immunostaining for Po in biopsied sural nerve from one family member with CMT1B was expressed in a small number of myelinated fibers. Immunoblot analysis for Po revealed that it was of normal molecular weight (29 kDa) although significantly reduced in amount. This heterozygous mutation could lead to a reduction in the total amount of normal protein in peripheral nerves through a mechanism of loss of function.