Methylation of cytosine residues is an almost ubiquitous finding of higher organisms (1 ). The majority of this methylation occurs at the dinucleotide CpG (where p denotes a phosphate group) (2 ). CpG sites are distributed throughout the genome with clusters of the sequence being found in the 5′ promoter region of housekeeping genes, in groups known as CpG islands. These short stretches of DNA have a have a C and G base composition, which in mammals and avians is estimated to be 10 times higher than in bulk DNA (3 )