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.
Genes may also have distant promoters (distal promoters) and these frequently contain CpG islands as well.  An example is 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.