It has become apparent that the genomes of many organisms are characterized by unique patterns of DNA methylation which can differ from genome segment to genome segment and cell type to cell type. These patterns can be instrumental in determining cell type and function. Thus, it is not surprising that studies on the role of DNA methylation now occupy center stage in many fields of biology and medicine such as developmental biology, genetic imprinting, genetic disease, tumor biology, gene therapy, cloning of organisms and others. Once again, basic research in molecular biology has provided the essential foundation for investigations of biomedical problems.