After exposure to UV light for 1 hour, both peptides selectively labelled BRD3-BD2 in the presence of BSA as observed by in-gel fluorescence of the FAM label after separation by SDS-PAGE. Control experiments were carried out without irradiation where no labelling was observed. Next, we evaluated labelling in cell lysate (HEK 293T) that had been spiked with purified recombinant BRD3-BD2 (Figure ). Pleasingly, we observed high levels of selectivity at a range of ligand concentrations (4, 2, 1 µM). Even at the highest concentration tested, we did not see any significant off-target background labelling. Consistent with our earlier experiments with the untagged peptides, these crosslinking experiments using the labelled peptides show Bpa-P1 is a moderately better crosslinker than Bpa-P2. Together the data with purified proteins and in lysate suggest that our peptides are highly selective for their protein target.