Histone demethylase that specifically demethylates dimethylated 'Lys-20' of histone H4 (H4K20me2), thereby modulating the chromosome architecture (PubMed:28867287). Promotes chromatin compaction by converting H4k20me2 to H4K20me1 leading to transcriptional repression (PubMed:28867287). Required for X chromosome dosage compensation by enriching H4K20me1 on X chromosomes and thereby reducing X-linked gene transcription in hermaphrodites throughout development (PubMed:14660541, PubMed:23028348, PubMed:3779843, PubMed:3478715, PubMed:26641248, PubMed:22393255, PubMed:28867287). X chromosome specificity is mediated by the recruitment through proteins of the condensin-like dosage compensation complex (DCC) (PubMed:14660541, PubMed:28867287). Required for the enrichment of H4K20me1 on autosomes in meiotic germ cells leading to their compaction in a DCC-independent mechanism (PubMed:28867287). Involved in 3-dimensional chromosome organization by strengthening the borders of topologically associating domains (PubMed:28867287). Involved in the regulation of growth, fecundity and body fat metabolism downstream of the TOR complex 2 and the protein kinase sgk-1 pathway (PubMed:23884442). Also involved in male tail development (PubMed:6537930). Binds 1 Fe(2+) ion per subunit. Interacts with the dosage compensation proteins dpy-27, dpy-26 and sdc-3; the interaction is probably involved in dpy-21 recruitment to the X chromosomes in hermaphrodites (PubMed:14660541). Interacts with the serine/threonine-protein kinase sgk-1 (PubMed:23884442). Specifically localizes to the X chromosomes around the 300- to 350-cell stage in hermaphrodite (XX) but not in male (X0) embryos (PubMed:14660541, PubMed:28867287). Requires sdc-2, sdc-3, dpy-26, dpy-27 and dpy-28 for X chromosome localization (PubMed:14660541). Colocalizes with X chromosomes during interphase, but is diffusely distributed during mitosis (PubMed:28867287). Localizes to all autosomes but not to X chromosomes in pachytene nuclei (PubMed:28867287). Expressed throughout development, with highest expression during embryogenesis. Mutant hermaphrodites exhibit low larval lethality, and survivors have a XX-specific shorter and stouter body morphology and are egg-laying defective (PubMed:14660541, PubMed:28867287). Leads to lethality in animals with three X chromosomes and to intersex development in males with a sex-chromosome to autosome (X:A) ratio higher than 0.65 (PubMed:6537930). Overexpression of X-linked gene transcripts (PubMed:23028348, PubMed:3478715, PubMed:26641248, PubMed:28867287). Increased binding of the RNA Pol II large subunit ama-1 to promoters on the X chromosome relative to autosomes (PubMed:26641248). Reduced levels of 'Lys-20' monomethylation (H4K20me1) and an increase of 'Lys-20' trimethylation (H4K20me3) and 'Lys-16' acetylation (H4K16ac) on histone H4 of hermaphrodite X chromosomes (PubMed:23028348, PubMed:22393255, PubMed:26641248, PubMed:28867287). Increased volume of the X chromosome (PubMed:28867287). Suppresses the XO-specific lethality in a xol-1 mutant background, where the DCC is inappropriately activated (PubMed:28867287). RNAi-mediated knockdown results in shortened lifespan (PubMed:23884442). In a sinh-1 single or a rict-1;sgk-1 double mutant background, suppresses the developmental delay phenotype (PubMed:23884442). In a rict-1 mutant background, suppresses the developmental delay, elevated body fat mass and low brood size, but shortens the lifespan and decreases the body size (PubMed:23884442). Belongs to the round spermatid basic protein 1 family.