Abstract

DNA methylation plays a role in the repression of gene expression in animal cells. In the mouse preimplantation embryo, most genes are unmethylated but a wave of de novo methylation prior to gastrulation generates a bimodal pattern characterized by unmethylated CpG island‐containing housekeeping genes and fully modified tissue‐specific genes. Demethylaton of individual genes then takes place during cell type specific differentiation, and this demodification may be a required step in the process of transcriptional activation. DNA modification is also involved in the maintenance of gene repression on the inactive X chromosome in female somatic cells and the marking of parental alleles at genomically imprinted gene loci.