We then moved to assess the degradation profile of 86 by western blot and live cell kinetics. Firstly, we treated HEK293 cells with varying concentrations of 86 and monitored intracellular levels of on-target BET, and off-target CRBN and VHL degradation (Figure , Table ). ). This hook effect can also be seen to a weaker extent in BRD4 Long blot (Figure ). The earlier onset of the hook effect is likely due to 86 inhibiting BRD2 and BRD4 more strongly due to its extra linkage to a second JQ1 molecule and potential BET bivalency. The switch in BET protein preference and increase in potency that 86 has for degrading BRD2 compared with the heterotrivalent compounds is likely due to the extra JQ1 "arm". Trivalent PROTAC SIM1, also shows preferential degradation of BRD2 by simultaneously engaging both BD1 and BD2 of the same BRD2 protein with high avidity, forming a stable 1:1:1 (BRD2 BD1-BD2 :SIM1:VHL) ternary complex with VHL. 11 This is likely the reason for the observed switch in selectivity, especially when comparing heterotetravalent PROTAC 86 to its heterotrivalent matched pair 26 (Table ).