Peripheral GLP-1 and GLP-1R agonists (GLP-1RA) used in the treatment of type 2 diabetes (T2D) to regulate hyperglycemia, readily diffuses across the blood–brain barrier (BBB) in animal models (Hunter and Holscher 2012), exerting positive effects on insulin secretion and glucose tolerance, at least in part, through CNS GLP-1R activation (Larsen et al. 1997). The gene discussed is GCG; the disease is type 2 diabetes mellitus.