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Science. 2004 Jan 30;303(5658):666-9.

Reactivation of the paternal X chromosome in early mouse embryos.

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X inactivation group, MRC Clinical Sciences Centre, ICSM, Hammersmith Hospital, London, W12 0NN, UK.


It is generally accepted that paternally imprinted X inactivation occurs exclusively in extraembryonic lineages of mouse embryos, whereas cells of the embryo proper, derived from the inner cell mass (ICM), undergo only random X inactivation. Here we show that imprinted X inactivation, in fact, occurs in all cells of early embryos and that the paternal X is then selectively reactivated in cells allocated to the ICM. This contrasts with more differentiated cell types where X inactivation is highly stable and generally irreversible. Our observations illustrate that an important component of genome plasticity in early development is the capacity to reverse heritable gene silencing decisions.

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