Cytosine methylation and mammalian development

Programmed methylation and demethylation of regulatory sequences has been proposed to play a central role, in vertebrate development. The methylation status of 5' regions of a panel of tissue-specific genes has not been correlated with expression in tissues of fetal and newborn mice despite efforts to do so. A model in which cytosine methylation is a biochemical specialization of large genomes and participates in specialized biological functions, among them allele-specific gene expression and heritable transcriptional silencing of parasitic sequence elements. Cellular differentiation is regulated by conserved regulatory networks. These networks do not depend on covalent modification of the genome.