inactivated. Artificially placing and expressing the Xist gene on another chromosome leads to silencing of that chromosome. Prior to inactivation, both X chromosomes weakly express Xist RNA from the Xist gene. During the inactivation process, the future Xa ceases to express Xist, whereas the future Xi dramatically increases Xist RNA production. On the future Xi, the Xist RNA progressively coats the chromosome, spreading out from the XIC; the Xist RNA does not localize to the Xa. The silencing of genes along the Xi occurs soon after coating by Xist RNA. Like Xist, the Tsix gene encodes a large RNA which is not believed to encode a protein. The Tsix RNA is transcribed antisense to Xist, meaning that the Tsix gene overlaps the Xist gene and is transcribed on the opposite strand of DNA from the Xist gene. Tsix is a negative regulator of Xist; X chromosomes lacking Tsix expression (and thus having high levels of Xist transcription) are inactivated much more frequently than normal chromosomes. Like Xist, prior to inactivation, both X chromosomes weakly express Tsix RNA from the Tsix gene. Upon the onset of X-inactivation, the future Xi ceases to express Tsix RNA (and increases Xist expression), whereas Xa continues to express Tsix for several days. Rep A is a long non coding RNA that works with another long non coding RNA, Xist, for X inactivation. Rep A inhibits the function of Tsix, the antisense of Xist, in conjunction with eliminating expression of Xite. It promotes methylation of the Tsix region by attracting PRC2 and thus inactivating one of the X chromosomes. Silencing The inactive X chromosome does not express the majority of its genes, unlike the active X chromosome. This is due to the silencing of the Xi by repressive heterochromatin, which compacts the Xi DNA and prevents the expression of most genes. Compared to the Xa, the Xi has high levels of DNA methylation, low levels of histone acetylation, low levels of histone H3 lysine-4 methylation, and high levels of histone H3 lysine-9 methylation and H3 lysine-27 methylation mark which is placed by the PRC2 complex recruited by Xist, all of which are associated with gene silencing. PRC2 regulates chromatin compaction and chromatin remodeling in several processes including the DNA damage response. Additionally, a histone variant called macroH2A (H2AFY) is exclusively found on nucleosomes along the Xi. Barr bodies DNA packaged in heterochromatin, such as the Xi,