The Xist gene maps to the X inactivation center region in both mouse and human, and previous analysis of the 3' end of the gene has demonstrated inactive X-specific expression, suggesting a possible role in X inactivation. We have now analyzed the entire mouse Xist gene. The mature inactive X-specific transcript is 15 kb in length and contains no conserved ORF. The Xist sequence contains a number of regions comprised of tandem repeats. Comparison with the human XIST gene demonstrates significant conservation of sequence and gene structure. Xist RNA is not associated with the translational machinery of the cell and is located almost exclusively in the nucleus. Together with conservation of inactive X-specific expression, these findings support a role for Xist in X inactivation, possibly as a functional RNA or as a chromatin organizer region.