This structure (PDB: 3B95) shows the ankyrin repeat domain of the histone methyltransferase GLP in complex with a histone H3 N-terminal peptide. While the SET domain catalyses methylation, the ankyrin repeat domain functions as a recognition module that binds methylated lysine 9 (H3K9). The tandem ankyrin repeats form a curved repeated arrangement of α-helices, creating a concave peptide-binding surface. The H3 peptide adopts an extended conformation along this surface, positioning the methylated lysine within a pocket formed by conserved aromatic residues. This pocket accommodates mono- and dimethylated lysine via hydrophobic and cation–π interactions, while excluding trimethylated lysine due to steric constraints. These features explain how GLP selectively recognises methylation states and contributes to transcriptionally repressive chromatin. This structure highlights how histone-modifying enzymes can also function as “readers,” linking molecular recognition to epigenetic regulation.