# CpG Islands CG-Islands are regions of DNA that have a higher frequency of containing the CG dinucleotide. - Sometimes referred to as *CpG-Islands* (where p=phosphate) > [!question] Why Are CG Dinucleotides So Rare? > > Despite the human genome’s GC-content being approximately 42%, the frequency of this dinucleotide is only observed to be 1%. > - This is significantly lower than the 4.41% that would be assumed using basic probability. > > This unique phenominon is caused by DNA methylation, the most common type of DNA modification. > > Following the addition of a methyl group ($CH_3$) to the cytosine nucleotide within a $CG$ dinucleotide, the resulting methylated cytosine often is *further deaminated into thymine*. Methylation has often been observed to be supressed in areas around genes known as CG-islands, where CG dinucleotides appear frequently. Because of this, it is very common to search for CG-islands when studying a new mamallian genome. ## Identification of CpG-Islands > See also: > - [[Hidden Markov Models (HMMs)]]