GLP1R and diabetes mellitus: PET imaging with tracers targeting GLP-1 receptors such as radiolabeled exendin-4 provides a noninvasive assessment of β-cell mass in the pancreas.152–154 Studies in both animals and humans have shown reduced tracer uptake in diabetes and insulin-resistant models, indicating a reduction in β-cell mass.155 Single-photon emission CT imaging with indium-111-labeled GLP-1 receptor probes156 has also shown decreased β-cell signal in these models, indicating a reduction in β-cell mass154 and possibly impaired insulin secretion.