DPP4 and diabetes mellitus: Currently, there are various drugs used to treat diabetes mellitus, including insulin secretagogues, insulin sensitizer, α-glucosidase inhibitor, insulin, or insulin analogues, dipeptidyl peptidase-4 (DPP-4) inhibitor, glucagon-like peptide-1 (GLP-1) analogues, and other oral antidiabetic drugs [3].