To confirm the ubiquitin-proteasome system's (UPS) involvement in the degradation of HDAC6 and HDAC10, we conducted rescue experiments using binding competitors and UPS inhibitors. In addition, we synthesized a non-degrading control AP1-N, which contains a methylated glutarimide ring (Scheme S1B, Supplementary Information), and thus cannot bind to CRBN. Since pretreatment with competitors can increase cytotoxicity, we reduced the treatment time to 6 h. Initial experiments without cotreatments confirmed that the reduced treatment time does not affect the degradation of class IIb HDACs by our PROTACs (Figure ). In the subsequent mode of action studies, we pretreated MM.1S cells for 0. Representative image of n = 2 biologically independent experiments, each performed in triplicates.