BTK kinase domain was diluted to 2 uM in buffer, and 3 uM PROTAC was added by adding 1/100 th volume from a 300 µM solution. The PROTACs were incubated with BTK at room temperature for various times. For testing by LCMS, 24 µl of the solution were mixed with 6 µl of 2.4% formic acid, and 10 µl were injected to LCMS.