HCRTR2 and Parkinson disease: We group these genes into several categories: (1) neurite outgrowth: SLITRK1; (2) histamine pathway: HDC; (3) serotonin pathway: SERT, HTR1A HTR2B; (4) glutamate pathway: SLC1A3; (5) synaptic signal transduction and cell-adhesion pathway: NLGN4, CDH2, CNTNAP2/CASPR2, DPP6; (6) mitochondrial functions: IMMP2L, MRPL3; and (7) genes associated with other diseases: DNAJC13 [Parkinson’s disease (PD)], OFCC1 (orofacial clefts), and HCRTR2 or OX2R (excessive daytime sleepiness).