To further investigate the mode of action and confirm the expected degradation mechanism, we conducted co-treatment experiments of activated 12 with the HDAC inhibitor vorinostat (SAHA, 1 µM), neddylation inhibitor MLN4924 (1 µM), CRBN ligand lenalidomide (5 µM), and the proteasome inhibitors MG132 (1 µM), and bortezomib (0.5 µM). In all co-treatment conditions, the HDAC6 levels increased (see Figure ), indicating that HDAC6 degradation requires the binding of 12 to both HDAC6 and CRBN and is neddylation-and proteasomedependent. To explore the differential activities of the cis-and trans-state of PHOTAC 12, we monitored ternary complex formation through TR-FRET between purified CRBN-DDB1ΔB and HDAC6.