The site of crosslinking was also determined for 3, 4 and 6, which were found to crosslink to a neighbouring aspartic acid residue, D104, suggesting a different orientation of the diazirine, but consistent with binding to the same pocket (Supplementary Figure ). The binding of PhABit 2 was further investigated by elucidation of a co-crystal structure of 2 with BRD4-BD1 (without UV irradiation). The structure showed the PhABit warhead binding in the acetyl lysine pocket, forming a hydrogen bond to asparginine N140, with the diazirine group oriented out of the pocket and directly adjacent to residue D103, consistent with the observed crosslinking to this residue (Figure ).