Histone demethylase that specifically demethylates dimethylated 'Lys-20' of histone H4 (H4K20me2), thereby modulating chromosome architecture. Binds 1 Fe(2+) ion per subunit. Testis. Expressed exclusively in haploid round spermatids. Expressed abundantly in the testis at 23 days after birth and later. Expressed exclusively in the germ cells. Phosphorylated by PKA. Belongs to the round spermatid basic protein 1 family.