DNA methylation denotes the addition of a methyl group to DNA, which in eukaryotes occurs predominantly at cytosines that are adjacent to guanine (CG). Because methylation does not alter the DNA sequence, it is referred to as an epigenetic mark. The sequence symmetry of the CG dinucleotide enables propagation of the methyl mark through cell division in a process that is mechanistically well understood . This inheritability makes DNA methylation highly attractive as a potential means to store information in a form of epigenetic memory that regulates genes over developmental processes or in response to environmental conditions. However, it has proven difficult to substantiate this function because it requires showing not only that a DNA methylation pattern coincides with a particular transcriptional state, but more importantly, that it controls it.