We incubated K562 or Mino cells with 1 µM of the compounds for 24 hours and measured the abundance of BTK by western blot (Supp. Fig. ). Several of the tested PROTACs displayed clear BTK degradation and could serve as potential starting points for optimization. We selected compound RC-1, which is based on a PEG6 linker and displayed consistent prominent levels of degradation in both cell lines, as a starting point for this study.