In a rare genetic disease Prader-Willi syndrome (PWS), caused by a deletion in the paternal chromosome 15 or by maternal uniparental disomy and characterized by weak muscle tone (hypotonia), poor growth, delayed development and chronic overeating, an impaired level of hypocretin-1 in the CSF is reported, which correlates with the severity of excessive daytime sleepiness (EDS) in these patients (Nevsimalova et al., 2005). The gene discussed is HCRT; the disease is Prader-Willi syndrome.