Here, we present structural and mechanistic studies that define a trans-labeling covalent molecular glue mechanism. We found that a novel series of BRD4 molecular glue degraders act by recruiting the CUL4-DCAF16 ligase to the second bromodomain of BRD4. To gain a deeper understanding into the potential off-target effects of more reactive degraders, we conducted streptavidin pulldowns of K562 lysate incubated with engineered MMH2-Biotin, a probe molecule that appends linker-biotin onto the second exit vector of MMH2.