Histone demethylase that specifically demethylates dimethylated 'Lys-20' of histone H4 (H4K20me2) and trimethylated 'Lys- 20' of histone H4 (H4K20me3) into monomethyl H4K20 (H4K20me1) thereby modulating chromosome architecture. May play a critical role in regulating H4K20 methylation during spermatogenesis. . This is the function of RSBN1 (round spermatid basic protein 1, Ensembl gene identifier ENSG00000081019).