Methylation of DNA cytosine bases plays important roles in the regulation of gene transcription. It has been estimated that 60 to 90% of the cytosines in CpG dinucleotides are methylated. An increase in methyl-CpG correlates with transcriptional silencing for the whole chromosome especially in developmentally regulated genes. Methylation is believed to lead to transcriptional silencing by multiple mechanisms including alteration of transcription factor binding to promoters and alteration in chromatin structure.