In this issue of Molecular Cell, Zhang et al. report the structure of a ternary 
complex between the SET domain histone methyltransferase DIM-5, its cofactor, 
and a histone H3 peptide. The insight gained from analysis of a key amino acid 
provides an exciting opportunity to dissect the possible functional meaning of 
mono-, di-, and trimethylation of histone lysine residues in vivo that will 
complement existing approaches in the quest to crack the histone methylation 
code.