One of the major advantages of LUXendins is that they can be used to visualize GLP1R in both live and fixed complex tissues. Pancreatic islets of Langerhans served as the testbed for the novel LUXendins, since they express GLP1R, which is predominantly localized to the beta cell compartment . Following one hour incubation with LUXendin492, LUXendin551 and LUXendin615, intense labeling was observed throughout the islet, with large gaps apparent (presumably representing the GLP1R-negative alpha cell compartment, which comprises ~20% of the rodent islet, as reported ) (Figure ). In all cases, labeling with the novel LUXendins could still be observed following formalin-fixation (Figure ), further expanding the utility of the novel LUXendins for protein identification together with immunohistochemistry. Confirming specificity, LUXendin492, LUXendin551 and LUXendin615 signals co-localized with specific GLP1R monoclonal antibody staining (Novo Nordisk 7F38, fully validated in GLP1R -/-tissue ) (Figure ).