HEK293T cells were treated for 1 h with probes 2j, 2k, or 2l (10 M), or DMSO vehicle. Competition-based experiments were also conducted where cells were pre-treated with parent hits 2d (40 µM), or 2e (40 µM) for 1 h, before addition of alkyne-containing probes 2j (10 M) or 2k (10 M), respectively. Following incubation, treated cells were lysed, conjugated with biotin-azide by Cu-click, and biotinylated proteins were enriched using NeutrAvidin beads. Enriched proteins were digested with LysC and trypsin, prior to analysis by LC-MS/MS (Fig. ).