Charcot–Marie-–ooth disease type 1A (CMT1A) is an autosomal dominant peripheral neuropathy associated with a large DNA duplication on the short arm of human chromosome 17. The trembler (Tr) mouse serves as a model for CMT1A because of phenotypic similarities and because the Tr locus maps to mouse chromosome 11 in a region of conserved synteny with human chromosome 17. Recently, the peripheral myelin gene Pmp–22 was found to carry a point mutation in Tr mice. We have isolated cDNA and genomic clones for human PM–22P. The gene maps to human chromosome 17p11.2–17p12, is expressed at high levels in peripheral nervous tissue and is duplicated, but not disrupted, in CMT1A patients. Thus, we suggest that a gene dosage effect involving PMP–22 is at least partially responsible for the demyelinating neuropathy seen in CMT1A.