Xist transcription in embryonic stem cells.

Patterns of Xist RNA expression in female ES cells undergoing differentiation using RNA fluorescence in situ hybridization. The left panel shows that undifferentiated ES cells have two punctate Xist RNA signals, representing the presence of unstable Xist transcripts at the site of transcription on both (active) X chromosomes. The middle panel shows that, upon differentiation, Xist RNA from one of the two alleles becomes stabilized and coats the X chromosome that is to be inactivated in cis . The X chromosome that remains active continues to express Xist in its unstable form. The right panel shows that, in fully differentiated cells, Xist RNA coats the inactive X chromosome and the Xist gene on the active X has been silenced.