As the glucose homeostasis actions and enhancement of insulin signaling provided by GLP-1 are largely preserved in type 2 diabetes mellitus (T2DM), agents that activate the GLP-1 receptor (GLP-1R), a class B G protein–coupled receptor (GPCR) that mediates the action of GLP-1, have been developed for the treatment of T2DM; they appear to be well-tolerated, efficacious and widely used5–9. This evidence concerns the gene GCG and diabetes mellitus.