Overexpression of Suv4-20h1, the enzyme that creates H4K20me2 from H4K20me1, results in G2 arrest, consistent with a role for H4K20me1 in mitosis. The results suggest that the same lysine on H4K20 may, in its different methylation states, facilitate mitotic functions in M phase and promote chromatin compaction and cell cycle exit in quiescent cells.