In humans, about 70% of promoters located near the transcription start site of a gene (proximal promoters) contain a CpG island.   CpG islands are generally 200 to 2000 base pairs long, have a C:G base pair content >50%, and have regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide and this occurs frequently in the linear sequence of bases along its 5 → 3 direction.
Distal promoters also frequently contain CpG islands, such as the promoter of the DNA repair gene ERCC1, where the CpG island-containing promoter is located about 5,400 nucleotides upstream of the coding region of the ERCC1 gene.  CpG islands also occur frequently in promoters for functional noncoding RNAs such as microRNAs.