Until recently, the only covalent DNA modification known to occur in mammals was the methylation of cytosine in CpG dinucleotides (5-mC), which is generally associated with gene repression. The connection between aberrant DNA methylation (5-mC) and malignant transformation is well established, with cancer cells frequently showing global hypomethylation, with localized hypermethylation at promoters which control cell cycle, tumor suppressor, or DNA-repair genes.