The X-inactivation center

A summary of the known elements and regions in the X-inactivation center (Xic) thought to affect choice, counting and cis-inactivation during the initiation of X inactivation. Genes are shown in bold, with the direction of transcription indicated by the arrow. Brx (brain X-linked), Tsx (testis X-linked) and Cdx4 (Caudal-4) are genes that lie within the Xic that do not have, at least for the moment, any defined X-inactivation function. The 2.1(2)P region shows differential histone H4 hyperacetylation in undifferentiated female and male embryonic stem (ES) cells and has been suggested as a possible regulatory element in X inactivation. P1 and P2 are the somatic Xist promoters, and P0 is a postulated Xist promoter in undifferentiated ES cells and early embryos. S12 and S19 are ribosomal protein pseudogenes found 5' to Xist in the mouse Xic. The deletions used to dissect the function of different elements in the Xic are shown as black lines. Blue bars indicate the regions that have been implicated in specific functions. Effects on choice and counting have not so far been distinguished in the regions indicated by light bars. The terminal two Xist exons, lying within the 65-kb deletion, have no effect on counting or choice.