CpG Islands

CpG Islands are short sequences, rich in cytosine and guanine bases (C and G), spread across DNA. These sequences are around 100-1000bp. 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. Mathylation of CpG islands is associated with silencing.

References

Zachariah RM, Rastegar M (2012) Linking epigenetics to human disease and Rett
syndrome: the emerging novel and challenging concepts in MeCP2 research. Neural Plast
2012: 415825