Both X-ray crystal structures show the BRD of TRIM33β is canonical, with N1039 residing inside the KAc-binding pocket, unlike TRIM33α (Figure ), where the residue is located 6.2 Å away (NH 2 to NH 2 distance) outside of the pocket. The H3-mimicking peptides form very similar interactions with TRIM33β to those formed with TRIM33α (Figure ). The peptide acts as an extended β-sheet, with a significant number of backbone interactions observed. The necessity of unmodified H3R2 for effective binding is reflected by this residue forming interactions with a number of residues; the interaction with N886 is present in all structures. The same is true for unmodified H3K4, which interacts with D884, E887, D888 and the backbone carbonyl oxygen of N886. H3N5 hydrogen bonds to D898, and H3R8 forms interactions with H910 (Figure ).