Summary of DNA methylation at CpG islands and genes, or repetitive elements such as interspersed repeats and centromeric satellites. Although Dnmt1 and Dnmt3a/3b are involved in DNA methylation at both repeated sequences (zigzag lines) and genes (closed boxes) with/without CpG islands (open boxes), Dnmt1 and Dnmt3a/3b act in a different fashion; Dnmt1 functions as a maintenance methyltransferase both in repeated sequences and CpG islands, whereas Dnmt3 primarily functions in CpG islands. Lower table represents the methylation statuses at various genomic areas in the wild-type and Dnmt-deficient ES cells reported here or elsewhere (Okano et al. 1999; Chen et al. 2003).
