This NHP study demonstrates that oral sitagliptin doses that match doses that can be safely administered to humans [41, 43, 44] induce elevations in plasma and CSF levels of GLP-1 and GIP that align with the efficacy of sitagliptin in a well characterized model of PD in rodent [41, 43, 44]. The gene discussed is GCG; the disease is Parkinson disease.