PMP22 and hereditary neuropathy with liability to pressure palsies: PMP22 encodes for the 22-kD peripheral myelin protein 22, a tetraspan membrane protein in peripheral myelin, and the spectrum of PMP22-related disorders (loss of one allele causes HNPP, and the presence of three alleles causes CMT1A) illustrates the importance of the correct PMP22 dosage in myelin homeostasis.