DNA methylation and CpG suppression.

Cytosine methylation in vertebrate genomes occurs predominantly at the dinucleotide CpG. This dinucleotide is deficient in vertebrate DNA, an observation which has hitherto been explained by passive deamination of S-methylcytosine to thymidine. Since the frequency and distribution of CpG may prove to be a useful indirect means to study the function of DNA methylation, it is of interest that the observed 'CpG suppression' is less apparent within and around coding sequences. A variety of different mechanisms now appear to be responsible for maintaining a relatively high CpG level in these regions despite the apparent attendant disadvantage of mutation.