Figure 1

Bypassing paternal imprints to generate bi-maternal mice. On mouse chromosome 7, a paternal DNA methylation imprint (Me/blue circle) represses the ICE and allows expression of Igf2 from the paternal chromosome in normal diploid embryonic cells (arrow). Igf2 is not expressed from a maternal chromosome that has an active unmethylated ICE (lollipop). A similar but opposite situation occurs in a neighboring imprinted cluster on this chromosome, where expression of Cdkn1c depends on a maternally methylated ICE (Me/red circle). Note that chromosome 7 contains only one of the two normally paternally methylated ICEs deleted to generate bi-maternal mice [7••]. DNA methylation imprints on ICEs are erased in primordial germ cells of the developing gonad and in females these imprints are reacquired during oocyte maturation. Chromosomes in immature oocytes lack maternal ICE imprints and, if they also genetically lack Pat-ICEs (oblique rectangle) that are normally modified by paternal gametic DNA methylation, then this haploid chromosome set will have a maternal origin with the imprinted expression pattern of the paternal genome [7••].