The structural and functional importance of the correct patterns of DNA methylation in all parts of a mammalian genome is, unfortunately, not well understood. The stability, inheritability, and developmental flexibility of these patterns all point to a major role that these patterns appear to play in determining structure and funtion of the genome. Up to the present time, studies on the repetitive sequences, which comprise more than 90 per cent of the DNA sequences in the human or other genomes, have been neglected.