CpG islands are typically defined as short DNA regions of genome containing a high frequency of CG dinucleotides.11,12 CpG islands are often located in promoter regions in the 5' flanking region of housekeeping genes and many tissue-specific genes.11,13,14 Mammalian genome projects have identified approximately 15,500 and 29,000 CpG islands in the mouse and human genomes, respectively.15–18 Methylation of DNA at CpG dinucleotides is an epigenetic modification that plays an important role in the control of gene expression19 and chromosomal structure in mammalian cells.20 Cytosine methylation of the CpG dinucleotide sequence contributes significantly to genomic imprinting8,21 and X-chromosome inactivation,22 and plays a crucial role in the earliest stages of embryogenesis.23 DNA methylation is also a fundamental aspect of programmed fetal development,24 determining cell fate, pattern formation, and terminal differentiation.8
