Fig. 1.

XIST and X-inactivation. a Map of the human X-inactive-specific transcript (XIST) gene indicating the positions of seven distinct tandem repeat regions (colored boxes), most of which are conserved and thought important to XIST RNA function. Grey boxes represent the eight exons of XIST. b The XIST RNA territory localizes over the condensed DNA on the inactive X-chromosome (Barr body), and is marked by repressive epigenetic modifications, shown in human Tig-1 fibroblasts. Top panel: Staining for trimethyl histone H3K27 (H3K27me3, red) and XIST RNA (green) on the Barr body, (DAPI, blue, shown in black and white). Bottom panel: Staining for ubiquitinated histone H2A (green) over the Barr body (DAPI, blue). Scale bars, 5 µm. c Ideogram of an unbalanced X:9 translocation of a woman carrying an extra copy of chromosome 9 translocated to the long arm of X-chromosome. The black box marks the distal region on one chromosome 9 p-arm that was missing