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Imprinted X inactivation maintained by a mouse Polycomb group gene. Jianbo Wang, Jesse Mager, Yijing Chen, Elizabeth Schneider, James C. Cross, Andras Nagy, Terry Magnuson The Nature Genetics, July 2001, 28: 371 - 375. In mammals, dosage compensation of X-linked genes is achieved by the transcriptional silencing of one X chromosome in the female. This process, called X inactivation, is usually random in the embryo proper. In marsupials and the extra-embryonic region of the mouse, however, X inactivation is imprinted: the paternal X chromosome is preferentially inactivated whereas the maternal X is always active. Having more than one active X chromosome is deleterious to extra-embryonic development in the mouse. The gene eed (embryonic ectoderm development), a member of the mouse Polycomb group (Pc-G) of genes, is required for primary and secondary trophoblast giant cell development in female embryos. Results from mice carrying a paternally inherited X-linked green fluorescent protein (GFP) transgene implicate eed in the stable maintenance of imprinted X inactivation in extra-embryonic tissues.