Histone methyltransferase that specifically monomethylates 'Lys-20' of histone H4. H4 'Lys-20' monomethylation is enriched during mitosis and represents a specific tag for epigenetic transcriptional repression. Mainly functions in euchromatin regions, thereby playing a central role in the silencing of euchromatic genes. Required for cell proliferation, possibly by contributing to the maintenance of proper higher-order structure of DNA and chromosome condensation during mitosis. L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine Specifically localizes to mitotic chromosomes. Associates to chromatin-dense and transcriptionally silent euchromatic regions. Present in ovary, early embryos and throughout the development (at protein level). Deposed in the egg during oogenesis. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. PR/SET subfamily.