n 1991, Charcot-Marie Tooth (CMT) disease was the first autosomal dominant disease associated with a gene dosage effect due to an inherited DNA rearrangement. Most cases of CMT1A are associated with a 1.5-Mb tandem duplication in 17p11.2-p12, mediated by flanking segmental duplications, that encompasses the PMP22 gene (see Figure 1). The disease phenotype results from having three copies of the normal gene. The reciprocal product of the recombination, a single copy of the PMP22 gene, results in the clinically distinct hereditary neuropathy with liability to pressure palsies (HNPP) [3] .