CpG islands are associated with genes, particularly housekeeping genes, in vertebrates. CpG islands are particularly common near transcription start sites, and may be associated with promoter regions. Normally a C followed immediately by a G (a CpG) is rare in vertebrate DNA since the C's in such an arrangement tend to be methylated. This methylation helps distinguish the newly synthesized DNA strand from the parent strand, which aids in the final stages of DNA proofreading after duplication. However over evolutionary time methylated C's tend to turn into T's because of spontanious deamination. The result is that CpG's are relatively rare unless there is selective pressure to keep them or a region is not methylated for some reason, perhaps having to do with the regulation of gene expression. CpG islands are regions where CpG's are present at significantly higher levels than is typical for the genome as a whole. The CpG islands displayed in this browser are all also at least 200 bases long, and have a GC content of at least 50%.