Low hypocretin-1 levels in the CSF have been shown to correlate with hypothalamic hypocretin cell loss in narcolepsy and other forms of hypersomnia; therefore, it has been proposed that degenerative damage to hypocretin neurons (such as in PD) may be detected by low CSF hypocretin-1 concentrations and may also explain the sleep symptoms experienced by some PD patients [163]. This evidence concerns the gene HCRT and Parkinson disease.