A large number of studies have documented the methylation patterns of individual genes and repeated sequences in different tissues of various mammalian species. This has revealed a consistent picture of DNA modification where each cell type has its characteristic pattern. Tissue-specific genes are undermethylated in their cell type of expression but fully modified in other cells, while housekeeping genes contain 5′ CpG islands which are constitutively unmethylated in all cells (see Yeivin and Razin, pp. 523–568). Experiments in tissue culture have clearly demonstrated that these somatic patterns are fixed and conservatively passed on from generation to generation (Stein et al., 1982; Wigler et al., 1991), but little is known about how this modification profile is established during early stages of embryo development.