Histone-modifying enzymes play a critical role in modulating chromatin dynamics. In this report we demonstrate that one of these enzymes, PR-Set7, and its corresponding histone modification, the monomethylation of histone H4 lysine 20 (H4K20), display a distinct cell cycle profile in mammalian cells: low at G 1 , increased during late S phase and G 2 , and maximal from prometaphase to anaphase. The lack of PR-Set7 and monomethylated H4K20 resulted in a number of aberrant phenotypes in several different mammalian cell types. These include the inability of cells to progress past G 2 , global chromosome condensation failure, aberrant centrosome amplification, and substantial DNA damage. By employing a catalytically dead dominant negative PR-Set7 mutant, we discovered that its mono-methyltransferase activity was required to prevent these phenotypes. Importantly, we demonstrate that all of the aberrant phenotypes associated with the loss of PR-Set7 enzymatic function occur independently of p53. Collectively, our findings demonstrate that PR-Set7 enzymatic activity is essential for mammalian cell cycle progression and for the maintenance of genomic stability, most likely by monomethylating histone H4K20. Our results predict that alterations of this pathway could result in gross chromosomal aberrations and aneuploidy.Dynamic alterations in chromatin structure are modulated, in part, by the post-translational modifications of the DNAassociated histone proteins. Specialized chromatin-modifying enzymes can phosphorylate, acetylate, ubiquitylate, or methylate specific amino acids within certain histones, and each of these modifications are associated with distinct biological events (1). One of the first histone modifications to be identified nearly forty-five years ago was the methylation of histone H4 lysine 20 (H4K20) 4 (2). Earlier biochemical studies linked H4K20 methylation to diverse biological events including transcriptional regulation, chromatin compaction, cell division, and the formation of heterochromatin (3-9). Importantly, it was also found that H4K20 is differentially methylated in vivo and therefore can be either mono-, di-, or trimethylated (10). Together, these findings strongly suggest that different methylated states of H4K20 may be involved in distinct biological processes, similar to what is observed for the various methylated states of histone H3 lysine 4 and 9 methylation (11, 12).Increasing evidence indicates that certain enzymes are responsible for the specific degree of histone lysine methylation (13). For example, the mono-and dimethylation of histone H3 lysine 9 in humans is mediated by the G9a enzyme, whereas trimethylation is mediated by the SUV39H1 enzyme (14,15). Similarly, the Suv4 -20 enzymes are responsible for di-and trimethylation in mammals (16,17). Trimethylated H4K20 is associated with repressed chromatin because it is targeted to constitutive heterochromatin, various repetitive elements, and imprinting control regions (16,18,19). Dimethylated H4K40 is more widely distributed within...