CD200 and narcolepsy: While amino acid sequence variants of human OX1 and OX2 receptors (Figure 1) have not been implicated in human narcolepsy [1,10], genetic variants of the OX1 and OX2 have been inconsistently and weakly associated with many CNS disorders (Table l, Table 2) including sleep–wake dysregulation, polydipsia in schizophrenia [13,14], panic disorder [15], mood disorders [12], migraine [11,16] and cluster headache [17,18,19,20,42].