We also explored whether covalent crosslinking could be detected via a competition flow cytometry experiment. As before, incubation of HEK293T cells expressing FPR1 with Tracer-FITC gave rise to a population of fluorescent cells, which decreased in intensity upon co-incubation with Probe-TAMRA (Figure ). Sequential treatment of cells with Probe-TAMRA, irradiation with UV light, washing, and incubation with Tracer-FITC gave cells with decreased fluorescence (Figure ); a further decrease in signal was seen when a second probe-TAMRA incubation and irradiation step was included before Tracer incubation. A concomitant increase in fluorescent FPR1 labelling was seen on a gel with repeated irradiations (Supp. Figure ). Following background subtraction of the flow cytometry data, a drop in fluorescence intensity of 23% can be calculated for a single round of irradiation, suggesting that around a quarter of available FPR1 sites are crosslinked. Crosslinking yields for alkyl diazirines can be low, in part because they can form less reactive diazo species when irradiated in addition to the desired carbenes . Our data show that we can detect the irreversible removal of FPR1 binding sites from the cell surface using our photocrosslinking probe, and increase crosslinking yield by multiple rounds of probe incubation and irradiation.