X chromosome inactivation is mediated by Xist RNA stabilization

Low level Xist expression can be detected from both active X chromosomes (Xa) in female embryonic stem cells prior to X inactivation. After differentiation, Xist is expressed at high levels only from the inactive X chromosome (Xi). Differentiating female cells increase Xist expression from the Xi prior to silencing low level Xist expression from the Xa. The transition from low level to high level expression is regulated by the stabilization of Xist transcripts at the Xi. We suggest that these developmentally modulated changes in Xist expression are regulated by several different mechanisms: factors that stabilize Xist transcripts at the Xi, an activity that blocks this stabilization at the Xa, and a mechanism that silences low level Xist expression from the Xa.