As expected, the N980F mutant was more effective than N980A at reducing binding of the H31-27K18Ac and H31-27K9Me3K18Ac peptides to the TRIM24 PHD-BRD (Table ). The N1039F mutation also reduced binding of the H31-27K18Ac peptide to for the TRIM33β PHD-BRD, which has the canonical BRD. However, this mutation had no effect on the binding of the H31-27K18Ac and H31-27K9Me3K18Ac peptides to the TRIM33α PHD-BRD. This result is consistent with the structural data showing that N1039 is located outside of the KAc binding pocket of the BRD. This result is particularly interesting when considered together with the previous AlphaScreen and ITC data (Tables and). The ability of the TRIM33α PHD-BRD to bind to the H31-27K18Ac peptide, and the fact that this binding is not disrupted by the N1039F mutation, indicates that the K18Ac residue does bind in the BRD, and that N1039 does not move into the KAc binding pocket to interact with K18Ac. This suggests that K18Ac forms interactions with the TRIM33α BRD that do not involve N1039, which have not been previously identified.