The PWWP domain is named after a conserved Pro-Trp-Trp-Pro motif. The domain binds to Histone-4 methylated at lysine-20, H4K20me, suggesting that it is methyl-lysine recognition motif. Removal of two conserved aromatic residues in a hydrophobic cavity created by this domain within the full-length protein, Pdp1, abolishes the interaction o f the protein with H4K20me3. In fission yeast, Set9 is the sole enzyme that catalyzes all three states of H4K20me, and Set9-mediated H4K20me is required for efficient recruitment of checkpoint protein Crb2 to sites of DNA damage. The methylation of H4K20 is involved in a diverse array of cellular processes, such as organising higher-order chromatin, maintaining genome stability, and regulating cell-cycle progression.