Remarkable advances have recently elucidated the molecular genetic basis of inherited peripheral neuropathies. These studies revealed a novel mutational mechanism of a large DNA duplication as a cause for a common autosomal dominant demyelinating neuropathy. A peripheral nerve myelin gene, PMP22, located within the duplication is responsible for the demyelinating neuropathy by virtue of a gene dosage effect. The identification of PMP22 and other genes involved in myelinopathies demonstrate that these diseases represent a spectrum of disorders resulting from defects in myelin structure, maintenance, and/or formation.