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In Chemistry, Methylation is the addition of a methyl group to a compound.

In Epigenetics, Methylation in genetics refers the addition of a methyl group to a cytosine in DNA to convert it to 5-methylcytosine. Methylation occurs at any CpG sites, which are sequences of DNA where cytosine lies next to guanine. The process of methylation is mediated by an enzyme known as DNA methyltransferase[?]. CpG sites are quite rare in a eukaryotic genome except in regions near the promoter of a eukaryotic gene. These regions are known as CpG islands, and the state of methylation of these CpG sites are critical for gene activity/expression.

In early development (fertilisation to 8-cell stage), the eukaryotic genome is demethylated. From the 8-cell stage to the morula, de novo methylation of the genome occurs, modifying and adding epigenetic information to the genome. By blastocyst[?] stage, the methylation is complete. The importance of methylation was shown in knockout mutants without DNA methyltransferase. All the resulting embryos died at the morula stage.

The pattern of methylation has recently become an important topic for research. Studies have found that in normal tissue, methylation of a gene is mainly localised to the coding region, which is CpG poor. In contrast, the promoter region of the gene is unmethylated, despite a high density of CpG islands in the region.

Interestingly, in cancer cells, methylation is very high even in the promoter region, raising interest in the role of methylation in the induction of cancerous properties. Furthermore, the pattern of methylation has been shown to be a reliable marker of cancerous tissue, with a heavily methylated gene found in 90% or more patients with prostate cancer.

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