Histone methyltransferase that specifically monomethylates 'Lys-20' of histone H4 (H4K20me1) (PubMed:23028348). H4K20me1 is enriched on hermaphrodite X chromosomes and during mitosis (PubMed:23028348, PubMed:22393255). Involved in dosage compensation by repression of X-linked gene expression in hermaphrodites (PubMed:23028348). Plays a role in growth and body fat regulation downstream of the TOR complex 2 pathway (PubMed:23884442). L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine In embryos, it is expressed ubiquitously. In late embryos, it is expressed in hypodermal seam cells. In L3 and L4 larvae and thereafter, it is expressed in vulval precursor cells. In adult males, it is also expressed in 6 unidentified posterior cells. Highly expressed in eggs, then decreases. Mutant animals lack methylation of 'Lys-20' of histone H4 (H4K20me) (PubMed:23028348). In a glp-1(e2141) mutant background which lacks a germline, the X-linked genes aco-1, ajm-1 and apl-1 are up-regulated (PubMed:23028348). RNAi-mediated knockdown leads to embryonic lethality in a mutant background of the dosage compensation proteins dpy-21 or dpy-28 (PubMed:23028348). Increases 'Lys-16' acetylation of histone H4 on hermaphrodite X chromosomes (PubMed:22393255). In the TOR complex 2 mutant background rict-1, suppresses the growth delay and elevated body fat index (PubMed:23884442). Causes mitotic chromosome segregation defects and chromosome bridges resulting in delayed or arrested embryonic development and embryonic lethality (PubMed:28867287). Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. PR/SET subfamily.