For many years histone methylation was thought to be irreversible, due to the fact that the half-life of the histone methylation was approximately equal to the half-life of histones themselves. In 2004, Shi et al. published their discovery of the histone demethylase LSD1 (later classified as KDM1A), a nuclear amine oxidase homolog. Since then many more histone demethylases have been found. Defined by their mechanisms, two main classes of histone demethylases exist: a flavin adenine dinucleotide (FAD)-dependent amine oxidase, and an Fe(II) and α-ketoglutarate-dependent hydroxylase. Both operate by hydroxylation of a methyl group, followed by dissociation of formaldehyde. Demethylation has implications for epigenetics.