Progress in the elucidation of the genetic basis for inherited peripheral neuropathies has been remarkable over the last years. In particular, the molecular mechanisms underlying the autosomal dominantly inherited disorders Charcot-Marie-Tooth disease type 1A (CMT 1 A), Charcot-Marie-Tooth disease type 1B (CMTlB), and hereditary neuropathy with liability to pressure palsies (HNPP) have been determined. While mutation in the gene encoding the major myelin protein, Po has been associated with CMTlB, C M T l A and HNPP have been shown to be associated with reciprocal recombination events leading either to a large submicroscopic duplication in CMTlA, or the corresponding D N A deletion in HNPP. Available evidence is consistent with the hypothesis that one or more genes within the relevant rearranged segment of 1.5 Mb on chromosome 17 is sensitive to gene dosage providing a novel mechanism for inherited human disorders. It is likely that the gene encoding the peripheral myelin protein PMP22 is at least one of the genes involved since the PMP22 gene maps within the C M T l A duplication (or HNPP deletion), and point mutations within it have been shown to cause a CMT phenotype in humans and comparable neuropathies in rodents (trembler and trembler'). The mechanism(s) by which gene dosage and point mutations affecting the same gene might lead to a similar phenotype are currently unknown but recent transgenic mouse experiments suggest that similar mechanisms may also underlie other genetic diseases.