Epigenetic reprogramming in mammalian development

DNA methylation is a major epigenetic modification of the genome that regul ates crucial aspects of its function. Genomic methylation patterns in somat ic differentiated cells are generally stable and heritable. However, in mam mals there are at least two developmental periods-in germ cells and in prei mplantation embryos-in which methylation patterns are reprogrammed genome w ide, generating cells with a broad developmental potential. Epigenetic repr ogramming in germ cells is critical for imprinting; reprogramming in early embryos also affects imprinting. Reprogramming is Likely to have a crucial role in establishing nuclear totipotency in normal development and in clone d animals, and in the erasure of acquired epigenetic information. A role of reprogramming in stem cell differentiation is also envisaged.