Crosslinking efficiency appeared similar at room temperature and 4 °C, and tight binding was evident from the fact that the crosslinking signal did not diminish when cells were washed before irradiation (Supp. Figure ). This is consistent with the long-residence time of formyl-peptide agonist ligands at FPR1. To further confirm that the fluorescent species was FPR1, we performed deglycosylation of the sample, which led to the expected decrease in molecular weight of the fluorescent and FLAG bands (Supp. Figure ). We could also pull-down of FPR1 via the FLAG-tag, further confirming a covalent probe-protein complex. The off-target band is observed in both experiments after deglycosylation (unaffected by PNGaseF) and as a fluorescent band that is not enriched by FLAG pull-down (Supp. Figure ), as expected.