We had previously reported that the mutations of the myelln Po gene were completely linked with Charcot—Marie—Tooth neuropathy type 1B (CMT1B) in two families. In this study we found a different mutation In another family with CMT1B. The mutation, a methlonine substitution for Isoleucine at amino acid position 30, Is located in the extracellular domain, which constitutes an Immunoglobulin domain responsible for the function of Po as an adhesion molecule. The results confirmed that Po Is a gene responsible for CMT1B.