DPP4 and type 2 diabetes mellitus: In the treatment of type 2 diabetes, there are largely used drugs that act, in part, by inhibiting the secretion or action of glucagon, such as glucagon-like peptide-1 (GLP-1) and GLP-1 receptor agonists, dipeptidyl peptidase-4 inhibitors (DPP-4), and metformin [17–19].