Figure 2.

Schematic representation of the role of CTCF binding to the imprinted control region (ICR) at the Igf2/H19 locus in mice. On the maternally transmitted allele, four CTCF sites are occupied and constitute a strong insulator that blocks the ability of downstream enhancers to activate Igf2 expression. On the paternal allele, CpG sites within the ICR are methylated and CTCF does not bind. There is no insulator activity and Igf2 expression is activated on this allele. Similar mechanisms are found at the human locus (see Bell and Felsenfeld 2000; Hark et al. 2000; Kanduri et al. 2000).