DPP4 and type 2 diabetes mellitus: As the circulating levels of both intact GLP-1 and GIP decline rapidly due to inactivation by the membrane-anchored enzyme peptidase, dipeptidyl peptidase-4 (DPP-4; also known as the lymphocyte cell-surface marker CD26), GLP-1 receptor (GLP-1R) agonists that mimic GLP-1 but are resistant to degradation by DPP-4 have been widely used in the treatment of type 2 diabetes [59,60,61].