DNA methylation is one major epigenetic modification that is
crucial for the development and differentiation of various cell
types in an organism (Li et al., 1992; Okano et al., 1999). In mammals,
DNA demethylation occurs in the whole-genome level after
fertilization, but not in some loci, such as intracisternal A particle
(IAP) (Daxinger and Whitelaw, 2010; Wu and Zhang, 2010).