To establish the labeling efficacy and specificity of the novel LUXendins, SNAP-GLP1R:CHO-K1 cells were incubated with each probe, before washing and orthogonal SNAP labeling with cell impermeable SBG-TMR or SBG-SiR . High-resolution confocal images showed predominantly membrane-localized LUXendin staining in SNAP-GLP1R:CHO-K1 cells, which overlapped with labeling of the SNAP-tag located on the GLP1R N-terminus (Figure ). No signal was detected in mock (non-transfected) CHO-K1 cell controls (Supplementary Figure ). LUXendins were also able to label stably-transfected SNAP_GLP1R:INS1 832/3 rat beta cells (Figure ), as well as native INS1 832/3, which endogenously express GLP1R (Figure ). Demonstrating high specificity, signal was absent in INS1 832/3 GL1PR -/-cells, CRISPR deleted for the GL1PR (Figure ). Of note, LUXendin492 and LUXendin615 staining was less 'clean' than LUXendin551, with some fluorescent signal present in the cytoplasm. We have previously reported a similar staining distribution for LUXendin555 (TMR) versus LUXendin645 (Cy5) , demonstrating a general preference toward cyanine-based dyes over their xanthene-based counterparts for cell labeling. Nonetheless, all the LUXendins tested clearly label membrane GLP1R. We next validated the LUXendins for use in widefield microscopy, which is widely available in most labs, serves to illustrate the robustness of labeling, and has the added advantage of allowing detection of near-infrared probes using cost efficient and fast switchable LED excitation and sensitive sCMOS detectors. As for confocal imaging, a similar pattern of LUXendin492, LUXendin551, LUXendin615 staining was seen, with the cyanine-based dye (LUX551) performing superiorly (Supplementary Figure ).