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. Expressed genes on the inactive X chromosome 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. Many of the genes which escape inactivation are present along regions of the X chromosome which, unlike the majority of the X chromosome, contain genes also present on the Y chromosome. These regions are termed pseudoautosomal regions, as individuals of either sex will receive two copies of every gene in these regions (like an autosome), unlike the majority of genes along the sex chromosomes. Since individuals of either sex will receive two copies of every gene in a pseudoautosomal region, no dosage compensation is needed for females, so it is postulated that these regions of DNA have evolved mechanisms to escape X-inactivation. The genes of pseudoautosomal regions of the Xi do not have the typical modifications of the Xi and have little Xist RNA bound. The existence of genes along the inactive X which are not silenced explains the defects in humans with abnormal numbers of the X chromosome, such as Turner syndrome (X0, caused by SHOX gene) or Klinefelter syndrome (XXY). Theoretically, X-inactivation should eliminate the differences in gene dosage between affected individuals and individuals with a normal chromosome complement. In affected individuals, however, X-inactivation is incomplete and the dosage of these non-silenced genes will differ as they escape X-inactivation, similar to an autosomal aneuploidy. The precise mechanisms that control escape from X-inactivation are not known, but silenced and escape regions have been shown to have distinct chromatin marks. It has been suggested that