We were interested to investigate the effect of the 17-amino acid deletion observed in TRIM33β on its BRD structure, and whether this alteration affected the orientation of N1039 in relation to the KAc binding pocket. To achieve this, we sought to obtain X-ray crystal structures of the TRIM33β BRD and PHD cassette, as X-ray structural data have only previously been reported for TRIM33α. The TRIM33β cassette (residues 888-1127) was expressed and co-crystallized with the PHD-binding region of the H3 peptide (H3 1-9 K9Ac) (PDB: 5MR8). We also obtained an X-ray crystal structure of XXXX construct bound to a peptide that does not occur naturally, H3 1-10 K10Ac, which possesses the first 9 residues of the H3 peptide (K9 unmodified) with a KAc residue added as the 10 th residue. This peptide was ordered in error, but the X-ray crystal structure with it bound still provides useful structural information and so we have included it here. Both 5MR8 and XXXX show the TRIM33β PHD-BRD cassette crystallizing with the H3-mimicking peptide bound to the PHD of one protein chain, through residues 1-8, with the K9Ac or K10Ac residue occupying the KAc binding pocket of an adjacent protein chain (Figure and). revealing that, in both cases, the H3-mimicking peptide binds to the PHD of one chain and the KAc-binding pocket of an adjacent protein.