The molecular biology of histone H4 lysine 20 (H4K20) methylation, like many 
other post-translational modifications of histones, has been the subject of 
intensive interest in recent years. While there is an emerging consensus linking 
H4K20me1, H4K20me2, and H4K20me3 to transcription, repair, and constitutive 
heterochromatin, respectively, the specific details of these associations and 
the biological mechanisms by which the methylated histones are introduced and 
function are now the subject of active investigation. Although a large number of 
methylases capable of methylating H4K20 have been identified and characterized; 
there is no known demethylase of H4K20, though the search is ongoing. 
Additionally, many recent studies have been directed at understanding the role 
of methylated H4K20 and other histone modifications associated with different 
biological processes in the context of a combinatorial histone code. It seems 
likely that continued study of the methylation of H4K20 will yield extremely 
valuable insights concerning the regulation of histone modifications before and 
during cell division and the impact of these modifications on subsequent gene 
expression.