Residue and degree-specific methylation of histone lysines along with other 
epigenetic modifications organizes chromatin into distinct domains and regulates 
almost every aspect of DNA metabolism. Identification of histone 
methyltransferases and demethylases, as well as proteins that recognize 
methylated lysines, has clarified the role of each methylation event in 
regulating different biological pathways. Methylation of histone H4 lysine 20 
(H4K20me) plays critical roles in diverse cellular processes such as gene 
expression, cell cycle progression and DNA damage repair, with each of the three 
degrees of methylation (mono-, di- and tri-methylation) making a unique 
contribution. Here we discuss recent studies of H4K20me that have greatly 
improved our understanding of the regulation and function of this fascinating 
histone modification.