from S-Adenosyl methionine (SAM) onto the lysine or arginine residues of the H3 and H4 histones. There are instances of the core globular domains of histones being methylated as well. The histone methyltransferases are specific to either lysine or arginine. The lysine-specific transferases are further broken down into whether or not they have a SET domain or a non-SET domain. These domains specify exactly how the enzyme catalyzes the transfer of the methyl from SAM to the transfer protein and further to the histone residue. The methyltransferases can add 1-3 methyls on the target residues. These methyls that are added to the histones act to regulate transcription by blocking or encouraging DNA access to transcription factors. In this way the integrity of the genome and epigenetic inheritance of genes are under the control of the actions of histone methyltransferases. Histone methylation is key in distinguishing the integrity of the genome and the genes that are expressed by cells, thus giving the cells their identities. Methylated histones can either repress or activate transcription. For example, while H3K4me2, H3K4me3, and H3K79me3 are generally associated with transcriptional activity, whereas H3K9me2, H3K9me3, H3K27me2, H3K27me3, and H4K20me3 are associated with transcriptional repression. Epigenetics Modifications made on the histone have an effect on the genes that are expressed in a cell and this is the case when methyls are added to the histone residues by the histone methyltransferases. Histone methylation plays an important role on the assembly of the heterochromatin mechanism and the maintenance of gene boundaries between genes that are transcribed and those that aren’t. These changes are passed down to progeny and can be affected by the environment that the cells are subject to. Epigenetic alterations are reversible meaning that they can be targets for therapy. The activities of histone methyltransferases are offset by the activity of histone demethylases. This allows for the switching on or off of transcription by reversing pre-existing modifications. It is necessary for the activities of both histone methyltrasnsferases and histone demethylases to be regulated tightly. Misregulation of either can lead to gene expression that leads to increased susceptibility to disease. Many cancers arise from the inappropriate epigenetic effects of misregulated methylation. However, because these processes are at times reversible, there is interest in utilizing their activities in concert with anti-cancer therapies. In X chromosome inactivation In female organisms, a sperm containing an X chromosome fertilizes the egg, giving the