DPP4 and diabetes mellitus: Since natural cyclic peptides are proven to inhibit various proteases, and to consider the beneficial aspects of cyclic peptides in comparison to small chemicals, like less toxicity, no accumulation in organs, and rapid degradation of peptides, we hypothesized that cyclic peptides from natural sources could be an alternative source of therapeutics to inhibit serine protease DPP4 for treatment of diabetes mellitus.