- Methylated cytosines are found primarily at CpG dinucleotides, but are also found at non-CpG sites.
   - Most 5mCs are highly conserved epigenetic marks and are mostly found at CpG sites, where 70-80% of cytosines are methylated.
   - If CpG sites located in a gene body are hypermethylated, gene expression is increased. 
   - If CpG sites located in promoters or enhancers are hypermethylated, these regions cannot be bound by transcription factors, leading to trasncriptional silencing.
- Non-CpG methylation is restricted to specific cell types.
   - e.g., pluripotent stem cells, oocytes, neurons, and glial cells.
   - Accumulation of methylation at non-CpG sites and CpG sites in neurons seems to be involved in development and disease etiology.

Methylation of CpG Sites

A CpG site on a genome is the term applied to cytosine and guanine appearing consecutively on the same strand of nucleic acid (i.e., cytosine-phosphodiester bond-guanine site). When CpG sites occur frequently in a region of the genome, the region is called a CpG island. These regions appear frequently at the start of promoters in eukaryotes and generally can indicate the presence of a coding region.

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CpG islands (or CG islands) are regions with a high frequency of CpG sites.

CpG Islands

Phillips, T. (2008) The role of methylation in gene expression. Nature Education, 1(1), 116. https://www.nature.com/scitable/topicpage/the-role-of-methylation-in-gene-expression-1070/

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