Significantly down-regulated DEGs in PD monkeys mainly encoded neuroprotective or neurogenesis proteins and could be involved in inflammation, neuronal damage, mitochondrial function impairment, and blood-brain/gut barriers, such as claudin domain containing 2 (CLDND2) encoding a tight junction protein component of blood-brain/gut barriers, adhesion G protein-coupled receptor G1 (ADGRG1), G protein-coupled receptor 68 (GPR68), protocadherin-1 (PCDH1), platelet-derived growth factor receptor beta (PDGFRB) and solute carrier family 5 member 3 (SLC5A3) (Fig. 3a). Here, PCDH1 is linked to Parkinson disease.