stylistic:tone, stylistic:style_transfer
CpG islands

In mammals, the only exception for this global CpG depletion resides in a specific category of GC- and CpG-rich sequences termed CpG islands that are generally unmethylated and therefore retained the expected CpG content. CpG islands are usually defined as regions with: 1) a length greater than 200bp, 2) a G+C content greater than 50%, 3) a ratio of observed to expected CpG greater than 0.6, although other definitions are sometimes used. Excluding repeated sequences, there are around 25,000 CpG islands in the human genome, 75% of which being less than 850bp long. They are major regulatory units and around 50% of CpG islands are located in gene promoter regions, while another 25% lie in gene bodies, often serving as alternative promoters. Reciprocally, around 60-70% of human genes have a CpG island in their promoter region. The majority of CpG islands are constitutively unmethylated and enriched for permissive chromatin modification such as H3K4 methylation. In somatic tissues, only 10% of CpG islands are methylated, the majority of them being located in intergenic and intragenic regions.