Silencers around Igf2 and H19. The Igf2 gene is expressed from the paternal and the H19 gene from the maternal chromosome. Both genes share enhancers located downstream of H19, with the mesoderm (M) enhancers further downstream than the endoderm (E) enhancers. On the paternal chromosome (a), Igf2 uses the enhancers and H19 is switched off, whereas on the maternal chromosome (b), H19 uses the enhancers and Igf2 is switched off. (Transcription from Igf2 and H19 is shown by arrows.) Two differentially methylated regions are shown, the DMD, upstream of H19, and DMR1 at Igf2, both of which are methylated (shown by closed circles) on the paternal chromosome and unmethylated (open circles) on the maternal chromosome. The ability of the endoderm enhancers to affect transcription of H19 and Igf2 is controlled by the DMD, upstream of H19: when the DMD is methylated (on the paternal chromosome), the enhancers activate the Igf2 promoters. When unmethylated (on the maternal chromosome) the DMD acts as a boundary element, preventing the enhancers from accessing the Igf2 promoters. Recent work [1,2,3] has shown that three regions act as tissue-specific silencers (active silencers are shown by black boxes) of either H19 or Igf2. One, at the DMD, silences H19 on the paternal chromosome, and two silence Igf2 on the maternal chromosome. One located midway between Igf2 and H19 silences Igf2 in muscle, and DMR1 silences Igf2 in other mesodermal tissues. When DMR1 is knocked out (c), the silencer activity is lost and Igf2 is expressed from the maternal chromosome as well as the paternal chromosome: it is no longer imprinted. It is postulated that normally either the mesodermal enhancers are able to circumvent the boundary but are prevented from activating the promoter by the silencer, or that the silencer and boundary elements interact, and stop the mesodermal enhancers from accessing the promoter. When the silencer is deleted, Igf2 can be expressed. Putative interactions between a mesoderm enhancer and promoters or silencers at DMRI are shown by lines and arrows.