Programming the X Chromosome

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Science  30 Jan 2004:
Vol. 303, Issue 5658, pp. 633-634
DOI: 10.1126/science.1094408

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To compensate for the fact that females have two X chromosomes but males have only one, each female cell has one X chromosome inactivated. In females, it is the paternal X chromosome that is preferentially inactivated in all placental cells, but not in cells that develop into the embryo. In their Perspective, Hajkova and Surani discuss new work ( Okamoto et al ., Mak et al.) showing that the cells that develop into the embryo have an inactivated paternal X at the start of embryogenesis, which becomes reactivated during the early stages of development.