Current view of the potential roles of proteins enriched on the inactive X chromosome.

X chromosome inactivation (X-inactivation) is triggered by X-linked noncoding Xist RNA, which is expressed asymmetrically from one of the two X chromosomes in females and coats it in cis to induce chromosome-wide silencing. Xist RNA is thought to play a role as a platform in recruiting proteins involved in gene silencing and heterochromatinization, which mediate serial changes in epigenetic modification of the chromatin. During the last two decades, many proteins have been shown to be enriched on the inactivated X chromosome in mouse and human. Although the biological significance of most of them for X-inactivation has not been fully established, extensive studies of these proteins should provide a better understanding of the molecular basis of how X-inactivation mediated by Xist RNA is regulated. Here, we review the potential roles of some of these proteins in the stepwise process of Xist RNA-mediated chromosome silencing.