For binding reversibility experiments, the PROTACs were incubated with BTK for 2 hours at room temperature, followed by addition of 40 µM Ibrutinib (by addition of 1/100 th volume of a 4 mM solution in DMSO). The samples were incubated with Ibrutinib at 37°C for various times and tested by LCMS as described before. For the noncovalent PROTAC NC-1, Ibrutinib was added to the complex at a concentration of 4 µM and incubated for 1 hour at room temperature.