<P> 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 . </P> <P> 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 . Additionally, a histone variant called macroH2A (H2AFY) is exclusively found on nucleosomes along the Xi . </P> <P> DNA packaged in heterochromatin, such as the Xi, is more condensed than DNA packaged in euchromatin, such as the Xa . The inactive X forms a discrete body within the nucleus called a Barr body . The Barr body is generally located on the periphery of the nucleus, is late replicating within the cell cycle, and, as it contains the Xi, contains heterochromatin modifications and the Xist RNA . </P> <P> A fraction of the genes along the X chromosome escape inactivation on the Xi . The Xist gene is expressed at high levels on the Xi and is not expressed on the Xa . Many other genes escape inactivation; some are expressed equally from the Xa and Xi, and others, while expressed from both chromosomes, are still predominantly expressed from the Xa . Up to one quarter of genes on the human Xi are capable of escape . Studies in the mouse suggest that in any given cell type, 3% to 15% of genes escape inactivation, and that escaping gene identity varies between tissues . </P>

Genes that are turned off like the inactive x’s (barr bodies) in female cells are