The transition between proliferation and quiescence is frequently associated 
with changes in gene expression, extent of chromatin compaction, and histone 
modifications, but whether changes in chromatin state actually regulate cell 
cycle exit with quiescence is unclear. We find that primary human fibroblasts 
induced into quiescence exhibit tighter chromatin compaction. Mass spectrometry 
analysis of histone modifications reveals that H4K20me2 and H4K20me3 increase in 
quiescence and other histone modifications are present at similar levels in 
proliferating and quiescent cells. Analysis of cells in S, G2/M, and G1 phases 
shows that H4K20me1 increases after S phase and is converted to H4K20me2 and 
H4K20me3 in quiescence. Knockdown of the enzyme that creates H4K20me3 results in 
an increased fraction of cells in S phase, a defect in exiting the cell cycle, 
and decreased chromatin compaction. 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.