Charcot-Marie-Tooth (CMT) syndrome describes a genetically and clinically heterogeneous group of polyneuropathies. Electrophysiologically, at least two types of CMT can be distinguished; CMT1 which has decreased nerve conduction velocities (NCV) and CMT2 which has normal or near normal NCV with decreased amplitudes. For CMT1, three gene loci (on chromosomes 1, 17 and the X chromosome) have been mapped. The locus on chromosome 17, CMT type 1A (CMT1A), is responsible for the most common form of CMT which has recently been shown to be associated with a large DNA duplication. Recent data demonstrates that the CMT1A phenotype results from an inherited DNA rearrangement and a gene dosage effect. The trembler (Tr) and allelic tremblerJ (TrJ) mice have been proposed as animal models for CMT. Tr has similar electrophysiological and neuropathological features to CMT1 patients and maps to mouse chromosome 11 in a region of conserved synteny with human chromosome 17p. Tr and TrJ have recently been shown to have different point mutations in regions encoding putative transmembrane domains of the myelin specific protein PMP-22. The human peripheral nerve-specific PMP-22 gene maps within the CMT1A duplication. PMP-22 is thus a candidate gene for CMT1A. This paper describes the molecular genetics of CMT1A and sural nerve pathology in CMT1A patients with the CMT1A duplication.