Duplication of a 1.5-Mb DNA segment on chromosome 17p11.2-12 encompassing the PMP22 gene causes Charcot-Marie-Tooth disease type 1 A (CMT1A)1,2, which is an autosomal dominant demyelinating neuropathy and the most common subtype of CMT. The gene discussed is PMP22; the disease is Charcot-Marie-Tooth disease.