DRD4 and narcolepsy: Of the common PD and narcolepsy genes, several were directly associated with PD and narcolepsy behavioral phenotypes such as locomotion (DRD2, DRD3, DRD4, GDNF, SLC18A2), sleep (DRD2, DRD3, GRIN2A, HTR2A, NLGN1), circadian processes (CACNA1C, DRD2, DRD3, DRD4, GRIN2, MAPK1, NLGN1, PPARGC1A,), circadian entrainment (CACNA1C, GRIN2A, MAPK1), learning (COMT, DRD1, DRD2, DRD3 GRIN2A) and memory (COMT, DRD1, DRD2, DRD3, GRIN2A, HTR2A).