Epigenetic modifications of the genome have long been known to exist. For example, all cells in the body share the same DNA; accordingly, there must be a mechanism whereby different genes are active in liver cells than, for example, brain cells. The process of cell specialization involves silencing certain portions of the genome in a manner specific to each cell. DNA methylation is a mechanism known to be involved in cell specialization. Another well-known example of DNA methylation involves X-inactivation in females. Because females carry two copies of the X chromosome, one must be inactivated. The silencing of one copy of the X chromosome involves DNA methylation. Genomic imprinting is another long-established principle known to involve DNA methylation. In genomic imprinting the expression of specific genes is determined by the parent of origin. For example, the copy of the gene inherited from the mother is silenced, while the copy inherited from the father is active (or vice versa). The silent copy is inactive through processes involving DNA methylation. These changes all involve epigenetic processes parallel to those currently attracting so
