Next we explored the specificity of our peptides by testing their ability to crosslink to other closely related bromodomains in the BET-family, which in addition to BRD3 consists of three other conserved members (BRD2, BRD4 and BRDT) each possessing a pair of tandem bromodomains. Interestingly, despite their high sequence similarities, we found that the peptide ligands crosslinked more efficiently to BRD3-BD2 than to the closely related BRD2-BD2 and BRD4-BD2. Further, no crosslinking was observed to the N-terminal BD of BRD3 (BRD3-BD1) (Figure ). Similar to results with BRD3-BD2, across the proteins that showed covalent modification, Bpa-P1 crosslinked to a greater extent than Bpa-P2.