HCRTR2 and Cluster headache: Several candidate genes for cluster headache have been investigated, including period circadian regulator 3 (PER3), circadian locomotor output cycles kaput (CLOCK), hypocretin receptor 2 (HCRTR2), calcium voltage-gated channel subunit alpha1 A (CACNA1A), alcohol dehydrogenase 4 (ADH4), nitric oxide synthase (NOS), and methylenetetrahydrofolate reductase gene (MTHFR).