Histone methyltransferase that specifically methylates monomethylated 'Lys-20' (H4K20me1) and dimethylated 'Lys-20' (H4K20me2) of histone H4 to produce respectively dimethylated 'Lys-20' (H4K20me2) and trimethylated 'Lys-20' (H4K20me3) and thus regulates transcription and maintenance of genome integrity (PubMed:28114273, PubMed:24049080, PubMed:15145825). In vitro also methylates unmodified 'Lys-20' (H4K20me0) of histone H4 and nucleosomes (By similarity). H4 'Lys-20' trimethylation represents a specific tag for epigenetic transcriptional repression (PubMed:15145825). Mainly functions in pericentric heterochromatin regions, thereby playing a central role in the establishment of constitutive heterochromatin in these regions (PubMed:15145825). KMT5B is targeted to histone H3 via its interaction with RB1 family proteins (RB1, RBL1 and RBL2) (PubMed:16612004, PubMed:15750587). Plays a role in myogenesis by regulating the expression of target genes, such as EID3 (PubMed:23720823). Facilitates TP53BP1 foci formation upon DNA damage and proficient non-homologous end-joining (NHEJ)-directed DNA repair by catalyzing the di- and trimethylation of 'Lys-20' of histone H4 (By similarity). May play a role in class switch reconbination by catalyzing the di- and trimethylation of 'Lys-20' of histone H4 (PubMed:28114273). N(6)-methyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine = H(+) + N(6),N(6)-dimethyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine N(6),N(6)-dimethyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine = H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine Inhibited by 6,7-Dichloro-N-cyclopentyl-4-(pyridin-4-yl)phthalazin-1-amine (A-196). A-196 is competitive with the histone peptide substrate H4K20me1 but non competitive with S-adenosyl-L-methionine. Homodimer (PubMed:24049080). Interacts with HP1 proteins CBX1, CBX3 and CBX5 (PubMed:15145825). Interacts with RB1 family proteins RB1, RBL1 and RBL2 (PubMed:15750587, PubMed:16612004). Interacts (via C-terminus) with FRG1 (PubMed:23720823). Associated with pericentric heterochromatin. CBX1 and CBX5 are required for the localization to pericentric heterochromatin. Partial muscle-specific knockout mice display several signs of muscular dystrophy including necrosis and an increased number of centrally nucleated myofibers. RNAi-mediated knockdown in C2C12 muscle cells causes reduced myogenic differentiation of the cells. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. Suvar4-20 subfamily. Truncated C-terminus.