Of note, labeling with the novel LUXendins was co-localized with both SNAP-GLP1R and specific monoclonal antibody staining, as expected given the previous thorough validation of LUXendin555, LUXendin645 and LUXendin651 stablemates . Moreover, no LUXendin signal could be detected in INS1 832/3 cells CRISPR-deleted for the GLP1R. These data also confirm that the Exendin4-S39C scaffold tolerates a range of fluorophores without significant effects on labeling or pharmacology. While some punctate staining was seen with non-cyanine dyes, this might reflect cleavage of fluorophore rather than GLP1R activation, since: 1) all LUXendins were potent antagonists; 2) no co-localization from intracellular signals were seen in SNAP-GLP1R cell systems; and 3) we previously showed that punctate LUXendin signal was not co-localized with GLP1R monoclonal antibody . We observed pronounced increases in performance of cyanine dyes (Cy3, Cy5 and Cy7) when compared to CF488, TMR, and CPY, most probably due to their molecular nature. Lastly, we omitted functionalization with Alexa Fluors, since previous efforts directed at SNAP-GLP1R did not lead to appreciable labeling, presumably because the presence of multiple negatively-charged sulfonate moieties interfere with target binding.