Very rapidly a link of the orexin/hypocretin system to the sleep disorder narcolepsy was established through experiments where the preproorexin gene was knocked out in mice (Chemelli et al., 1999) and a gene defect was discovered in the orexin/hypocretin type II receptors in a colony of dogs with genetically determined narcolepsy (Lin et al., 1999). The gene discussed is HCRT; the disease is narcolepsy.