A 94 KB GENOMIC SEQUENCE-3' TO THE MURINE XIST GENE REVEALS AN AT RICH REGION CONTAINING A NEW TESTIS-SPECIFIC GENE TSX

X chromosome inactivation in both mouse and human requires the presenc e of a cis acting locus, the X inactivation centre. This locus is thou ght to be involved in the initiation and spreading of the inactivation signal in early development. In order to increase our understanding o f the mouse X inactivation centre, a 94 kb region immediately distal t o the Xist gene has been sequenced and analysed for the presence of tr anscription units and/or potential cis acting regulatory elements. We have identified a novel gene, Tsx, lying 40 kb 3' from Xist. Tsx is ex pressed specifically in the testis and shows no convincing homology to proteins currently in the databases, A rat homologue, also X linked, has been isolated, The mouse and rat Tsx sequences are highly divergen t, suggesting that part of the X inactivation centre, including both X ist and Tsx are subject to relatively weak evolutionary constraints.