Xist has properties of the X-chromosome inactivation centre

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Abstract
X-chromosome inactivation is the process by which female mammals (with two X chromosomes) achieve expression of X-chromosomal genes equivalent to that of males (one X and one Y chromosome). This results in the transcriptional silencing of virtually all genes on one of the X chromosomes in female somatic cells. X-chromosome inactivation has been shown to act in cis and to initiate and spread from a single site on the X chromosome known as the X-inactivation centre (Xic). The Xic has been localized to a 450-kilobase region of the mouse X chromosome. The Xist gene also maps to this region and is expressed exclusively from the inactive X chromosome. Xist is unusual in that it appears not to code for a protein but produces a nuclear RNA which colocalizes with the inactive X chromosome. The creation of a null allele of Xist in embryonic stem cells has demonstrated that this gene is required for X inactivation to occur in cis. Here we show that Xist, introduced onto an autosome, is sufficient by itself for inactivation in cis and that Xist RNA becomes localized close to the autosome into which the gene is integrated. In addition, the presence of autosomal Xist copies leads to activation of the endogeneous Xist gene in some cells, suggesting that elements required for some aspects of chromosome counting are contained within the construct. Thus the Xist gene exhibits properties of the X-inactivation centre.