Requires iron(II). This entry describes a group of enzymes that demethylate N-methylated L-lysine residues at position 27 of histone H3 (H3K27). The enzymes are dioxygenases and act by hydroxylating the methyl group, forming an unstable hemiaminal that leaves as formaldehyde. They can act on tri- and di-methylated forms, but have no activity with the mono-methylated form.