There are two known classes of histone demethylases (HDM), differing in their reaction chemistry, coenzyme use and reaction products. Flavin-dependent HDMs act only on mono- and dimethylated lysines and produce hydrogen peroxide, whereas the Jumonji-containing HDMs are iron-dependent enzymes that can act on mono-, di- and trimethylated lysine side chains, and also on methylated Arginine residues. Both classes also produce formaldehyde as a product of the demethylation reaction. LSD1 (Lysine-specific demethylase 1) is a flavindependent amine oxidase and was identified as a subunit of different complexes and has been shown to be involved in transcriptional repression of genes through the demethylation of monomethyl Lys4 and dimethyl Lys4 Histone H3. LSD1 is also involved in transcriptional activation through the demethylation of monomethyl Lys9 and dimethyl Lys9 Histone H3. It is also able to demethylate both mono- and dimethyl Lys370 in the regulatory domain of the tumor suppressor p53.