The X inactivation center candidate region (Xic/XIC in mouse and human) is poorly characterized for the presence of transcription units. Only two conserved genes have been isolated to date, Xist/XIST and Cdx4/CDX4. The other known gene lying within this region, Tsx, has been identified so far only in rodents by analyzing the complete genomic sequence of a 94-kb region distal to Xist. Here, we report the characterization of an additional gene lying within this 94-kb sequenced region. Brx, for Brain X-linked gene, is a rare transcript preferentially expressed in the brain. It is normally X-inactivated in the mouse. Localisation of BRX, its human homolog has shown the gene to be located within the orthologous but inverted human CDX4-XIST segment. These results suggest that the gene order of the region encompassing the Cdx4-Xist interval in the mouse is similar in human. Comparison of the Xist-Brx and Brx-Cdx4 regions in mouse and human indicates that these intervals are three times longer in human than in mouse. BRX is a new potential candidate for one of the X-linked mental retardation syndromes mapped within the pericentromeric region of the human X Chromosome (Chr).