Necessary for DNA damage checkpoint activation. Required for the association of set9 with chromatin and subsequent methylation of H4K20. Associates with H4K20me1 to increase the concentration of set9 on chromatin to perform H4K20me3. H4K20me3 is mainly enriched at heterochromatin and is required for proper heterochromatin assembly. Interacts with set9 and histone H4K20me1. Associates with nucleosomes. The C-terminal region mediates association with set9, whereas the N-terminal PWWP domain recognizes H4K20me1.