Deamination is the removal of an amine group from a molecule.
Common deamination reactions
Cytosine
Spontaneous deamination is the hydrolysis reaction of cytosine into uracil, releasing ammonia in the process. This can occur in vitro through the use of bisulphite, which converts cytosine, but not 5-methylcytosine. This property has allowed researchers to sequence methylated DNA to distinguish non-methylated cytosine (shown up as uracil) and methylated cytosine (unaltered).
In DNA, this spontaneous deamination is corrected for by the removal of uracil (product of cytosine deamination and not part of DNA) and replacement with cytosine.
5-methylcytosine
Spontaneous deamination of 5-methylcytosine results in thymine and ammonia. In DNA, this reaction cannot be corrected because the repair mechanisms do not recognize thymine as erroneous (as opposed to uracil), and unless it affects the function of the gene, the mutation will persist. This flaw in the repair mechanism contributes to the rarity of CpG sites in the eukaryotic genome.