QUOTE : Professor Jaenisch and his colleagues created mice carrying a genetically engineered version of the X Inactivation Centre (an X Inactivation Centre transgene). This was 450kb in size, and included the Xist gene plus other sequences on either side. They inserted this into an autosome (non-sex chromosome), created male mice carrying this transgene, and studied ES cells from these mice. The male mice only contained one normal X chromosome, because they have the XY karyotype. However, they had two X Inactivation Centres. One was on the normal X chromosome, and one was on the transgene on the autosome. When the researchers differentiated the ES cells from these mice, they found that Xist could be expressed from either of the X Inactivation Centres. When Xist was expressed, it inactivated the chromosome from which it was expressed, even if this was the autosome carrying the transgene16. These experiments showed that even cells that are normally male (XY) can count their X chromosomes. Actually, to be more specific, it showed they could count their X Inactivation Centres. The data also demonstrated that the critical features for counting, choosing and initiation were all present in the 450kb of the X Inactivation Centre around the Xist gene.