Finally, we investigated whether Probe-TAMRA could covalently crosslink to FPR1 in cells. We used a bespoke UV LED device that can achieve highly efficient crosslinking in seconds, minimising sample heating. Transfected HEK293T cells were suspended with the probe, irradiated at 365 nm in this device, lysed and analysed by SDS-PAGE. Excitingly, we saw a diffuse fluorescent band at ~60 kDa in the presence of probe and only upon UV irradiation and receptor expression (Figure ). A weak signal at slightly higher molecular weight was detected in the non-transfected sample, indicating crosslinking to an additional protein. This off-target band is not observable in the +FPR1 lane, likely due to the intensity of the FPR1 band. There is a small amount of FPR1-independent binding in flow cytometry data (Figure ) so it is possible that this band corresponds to another protein that also binds the probe.