In addition, although drug treatments such as insulin, glucagon-like peptide-1 (GLP-1/Exendin-4), thiazolidinediones (TZD), dipeptidyl peptidase 4 (DPP-IV) inhibitors, sodium–glucose co-transporter-2 (SGLT2) inhibitors, and metformin are successful to provide the therapeutics in T1D or T2D [155,156,157,158,159], none of these therapeutics provide a “functional cure” for the diabetes, which means life-long drug treatment to ameliorate diabetes is required for the patients. Here, GCG is linked to diabetes mellitus.