Recent years have seen rapid progress towards understanding the molecular 
mechanisms involved in X chromosome inactivation (X inactivation). This progress 
has largely revolved around the discovery of the X inactive 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 transcript has no apparent protein-coding potential and is 
retained in the nucleus 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 developmental 
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 which underlies the establishment of 
appropriate X inactivation patterns and to determine how Xist RNA participates 
in the process of propagating inactivation in cis.