DNA methylation is a covalent modification that is critical for the regulation of gene expression in a wide variety of biological contexts (Jaenisch and Bird, 2003; Bergman and Cedar, 2013). Methylation of DNA on cytosines is an important mechanism for silencing gene expression, and cytosine demethylation is required for gene activation (Ito et al., 2011; He et al., 2011). DNA methylation plays important roles in a variety of cellular processes, including genomic imprinting , X-chromosome inactivation and regulation of gene expression (Bird 2002).