To test whether the formation of the covalent adducts is reversible and estimate the timescale of the exchange, we added 40 µM Ibrutinib to the samples after formation of the adducts and incubated at 37°C. For the acrylamides IR-1 and IR-2, no Ibrutinib adduct was observed even after 28 hours of incubation. However, only 80-85% of protein appeared to be labeled by the PROTACs (Supp. Fig. ). This may indicate that in fact IR-1/2 have stably labeled 100% of the protein, thereby preventing Ibrutinib binding, but dissociation during the separation or ionization process on the LC-MS may have generated the observed free protein peak.