X-CHROMOSOME INACTIVATION AND THE XIST GENE

Recent years have seen rapid progress towards understanding the molecu lar mechanisms involved in X chromosome inactivation (X inactivation). This progress has largely revolved around the discovery of the X inac tive specific transcript (Xist) gene, which is known now to represent the master switch locus regulating X inactivation. In adult cells Xist is transcribed exclusively from the inactive X chromosome. The transc ript has no apparent protein-coding potential and is retained in the n ucleus in close association with the domain occupied by the inactive X chromosome. It is thus thought to represent a functional RNA molecule which acts as the primary signal responsible for the propagation of X inactivation. Developmental regulation of Xist correlates with the de velopmental timing of X inactivation. Recent results have demonstrated that Xist is both necessary and sufficient for X inactivation. Goals for the future are to understand the mechanism of Xist regulation whic h underlies the establishment of appropriate X inactivation patterns a nd to determine how Xist RNA participates in the process of propagatin g inactivation in cis.