The enrichment of markedly changed G protein-coupled receptors (GPCR) signaling pathways, such as dopamine (DRD1/5 signaling) (Hisahara and Shimohama, 2011) and serotonin (HTR4/6/7 signaling) (Huot et al., 2011), which are both linked to PD, coincides with a dysregulation of these pathways. Here, HTR4 is linked to Parkinson disease.