CpG islands are commonly 200 to 2000 base pairs long, have a C:G base pair content >50%, and have frequent 5 → 3 CpG sequences.  About 70% of human promoters located near the transcription start site of a gene contain a CpG island.
Promoters located at a distance from the transcription start site of a gene  also frequently contain CpG islands.  The promoter of the DNA repair gene ERCC1, for instance,  was identified and located about 5,400 nucleotides upstream of its coding region.  CpG islands also occur frequently in promoters for functional noncoding RNAs such as microRNAs and Long non-coding RNAs (lncRNAs).