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. The gene discussed is PMP22; the disease is Charcot-Marie-Tooth disease type 1A.