Histone methylation plays an essential role in epigenetic regulation and has been thought to be an irreversible and stable modification of histones. However, several enzymes have recently been discovered to demethylate mono- and dimethylated lysine residues of histone H3 as well as monomethylated arginines via either amine oxidation or deimination, respectively. The JmjC domain-containing histone demethylase 1 (JHDM1), which is conserved from yeast to human, has been demonstrated to demethylate mono- and di- but not trimethylated H3 K36 via hydroxylation of the methyl moiety within the methylated lysine residue. This study broadens our understanding of different types of reaction mechanisms and cofactor requirements for a different category of histone demethylating machinery.