For many years histone methylation was thought to be a permanent modification. Very recently two families of histone demethylating enzymes were discovered.
The first was Lysine Specific Demethylase 1 (LSD1) which is an flavin-dependent monoamine oxidase which can demethylate mono- and di-methylated lysines, specifically histone 3, lysines 4 and 9 (H3K4 and H3K9). This enzyme cannot demethylate tri-methylated lysines and for a short while it was thought that tri-methylated lysines may indeed be permanent modifications.
In late 2005 the Jumonji domain-containing (JmjC) histone demethylases were discovered which are able to demethylate mono-, di-, or tri-methylated lysines thereby disproving the theory that histone methylation is permanent once and for all. Although this conclusion has since come into question.[1] Two specific JmjC histone demethylases are PHF8 and KIAA1718.
References
^ Webby CJ et al. Jmjd6 catalyses lysyl-hydroxylation of U2AF65, a protein associated with RNA splicing. 2009 Science 325(5936):90-3.