Adie (1926) first delineated narcolepsy as a separate and specific entity. It is a sleep disorder characterized by attacks of disabling daytime drowsiness and low alertness. The normal physiologic components of rapid eye movement (REM) sleep, dreaming and loss of muscle tone, are separated and also occur while the subject is awake, resulting in half-sleep dreams and episodes of skeletal muscle paralysis and atonia (cataplexy and sleep paralysis). Unlike normal sleep, that of narcolepsy often begins with REM activity and the time taken to fall asleep is shorter than normal. In contrast to animal models, human narcolepsy is not a simple genetic disorder. Most human cases of narcolepsy are sporadic and carry a specific HLA haplotype (Peyron et al., 2000). Familial cases are the exception rather than the rule, and monozygotic twins show only partial concordance (25 to 31%) (Mignot, 1998). Genetic Heterogeneity of Narcolepsy Additional narcolepsy loci have been mapped to chromosomes 4 (NRCLP2; 605841), 21q (NRCLP3; 609039), 22q13 (NRCLP4; 612417), 14q11 (NRCLP5; 612851), and 19p13.2 (NRCLP6; 614223). NRCLP7 (614250) is caused by mutation in the MOG gene (159465) on chromosome 6p22. Resistance to narcolepsy is associated with minor alleles of a SNP and a marker in the NLC1A gene (610259) on chromosome 21q22.