Before introducing a Cys mutant into the first bromodomain of BRD4 [BRD4(1)], we wanted to rule out the possibility that the native Cys residues, C125 and C136 (Figure and S1), would react with electrophilic fragments. To test this, we incubated two KAc-mimicking methylisoxazole-based reactive fragments (1 and 2, Figure ) with purified BRD4(1) WT . After 20 hours, no covalent labeling of the protein by either fragment was detected using LCMS, indicating that neither of the native cysteines are highly reactive nucleophiles (Figure ).