Fig. 2.

Transcription and imprinting control of the Peg3 domain. The schematic representation illustrates a potential mode of the Peg3-DMR and ECR18 in the co-regulation and mono-allelic expression (imprinting) of four genes in the mouse brain: the paternally expressed Peg3 and Usp29 and the maternally expressed Zim1 and Zim2. On the paternal allele, the Peg3-DMR becomes a dominant user for the shared enhancer ECR18 and out-competes the remaining two genes, Zim1 and Zim2. Thus, Peg3 and Usp29 are expressed from the paternal allele, while Zim1 and Zim2 become silent on the paternal allele. On the maternal allele, the Peg3-DMR is methylated, thus causing the repression of both Peg3 and Usp29. As a consequence, the two genes Zim1 and Zim2 can access ECR18 and be expressed from the maternal allele. However, the current model cannot explain the imprinting of the three remaining genes, APeg3, Zim3 and Zfp264, since some of these genes are expressed mainly in testis but not in brain.