Isolation of CpG Islands Using a Methyl-CpG Binding Column

Vertebrate genomes are globally heavily methylated at the sequence CpG, with the exception of short patches of GC-rich DNA of between 1–2 kb in size that are free of methylation, and these are known as CpG islands (see refs. 1 and 2 for reviews). In addition to distinctive DNA characteristics, CpG islands also have an open chromatin structure in that they are hyperacetylated, lack histone H1, and have a nucleosome-free region (3) . The major reason for interest in CpG islands is that they co-localize with the 5′ end of genes. Both promoter sequences and the 5′ parts of transcription units are found within CpG islands. It has been estimated that 45,000 (56%) of human genes and 37,000 (47%) of mouse genes are associated with a CpG island (4) and these include all genes that are ubiquitously expressed as well as many genes with a tissue-restricted pattern of expression (5 ,6 ).