Human narcolepsy/cataplexy is associated with reduced hypocretin (orexin) transmission. A common preprophypocretin (HCRT) polymorphism (-909C/T) was identified and tested in 502 subjects (105 trio families, 80 Caucasian narcolepsy cases, and 107 Caucasian control subjects). This polymorphism was not associated with the disease. The promoter and 5' untranslated (5'URT) regions of the HCRT gene (-320 to +21 from ATG) were also sequenced in 281 subjects. None of the subjects carried -22T, a rare 5'UTR polymorphism previously reported to be associated with narcolepsy. The HCRT locus is not a major narcolepsy susceptibility locus.