GCG and diabetes mellitus: Since the discovery that GLP-1 can improve insulin secretion and glycemic control in patients with type 2 diabetes mellitus (T2DM) in the 1990s (Nauck et al., 1993; Ahrén and Gutniak, 1997; Vella et al., 2000), GLP-1 receptor agonists (GLP1-RAs) have been widely used in clinic.