LOCALIZATION AND EXPRESSION ANALYSIS OF A NOVEL CONSERVED BRAIN EXPRESSED TRANSCRIPT, BRX BRX, LYING WITHIN THE XIC/XIC CANDIDATE REGION/

The X inactivation center candidate region (Xic/XIC in mouse and human ) is poorly characterized for the presence of transcription units. Onl y two conserved genes have been isolated to date, Xist/XIST and Cdx4/C DX4. The other known gene lying within this region, Tsx, has been iden tified so far only in rodents by analyzing the complete genomic sequen ce of a 94-kb region distal to Xist. Here, we report the characterizat ion of an additional gene lying within this 94-kb sequenced region. Br x, for Brain X-linked gene, is a rare transcript preferentially expres sed in the brain. It is normally X-inactivated in the mouse. Localisat ion of BRX, its human homolog has shown the gene to be located within the orthologous but inverted human CDX4-XIST segment. These results su ggest that the gene order of the region encompassing the Cdx4-Xist int erval in the mouse is similar in human. Comparison of the Xist-Brx and Brx-Cdx4 regions in mouse and human indicates that these intervals ar e three times longer in human than in mouse. BRX is a new potential ca ndidate for one of the X-linked mental retardation syndromes mapped wi thin the pericentromeric region of the human X Chromosome (Chr).