CpG Islands
CpG Islands are short sequences, rich in cytosine and guanine bases (C and G), spread across DNA[1]. These sequences are around 100-1000 bp. In mammals, most cytosine residues followed by a guanine residue are methylated, however, most 'C's in CpG Islands escape methylation, we call this hypomethylation. The methylation of CpG islands results in the switching off of transcription. Therefore, CpG islands enhance transcription. This is important in mammals as 60%-70% of protein-coding genes lack TATA and initiators causing transcription to occur at a lower rate.
In mammals, most C residues followed by a G are methylated to 5-methyl C. However, generally, C residues in CpG islands escape methylation (they are hypomethylated), this is important for protein function. Methylation of CpG islands is associated with silencing[2].