INS and diabetes mellitus: Clinically, exendin-4 is given to patients with type 2 diabetes mellitus as an adjunct to metformin, sulfonylurea or basal insulin (Inzucchi et al., 2015), and it is the exendin-4 addition to metformin and/or sulfonylurea that enables type 2 diabetes mellitus patients to achieve glycaemic control via improvement of β-cell function (Kendall et al., 2005).