Location of an X-inactivation Centre in Mouse and Human
Experiments utilising X-autosome translocations in or autosomal insertions into X chromosome material(") indicated that autosomal coat colour genes placed adjacent to X chromosome material could be inactivated resulting in coat colour variegation. For X-autosome translocations, only autosomal genes carried on one of the translocation products appear to be inactivated, suggesting the location of a limited region or centre from which X-inactivation may be intitiated. Detailed studies of unbalanced embryos carrying the T16H X-autosome translocation have indicated that only the 16x translocation product can be inactivated('') and this, along with information from other X-autosome translocations, has indicated the presence of an X-inactivation centre distal to T16H and proximal to the T6Rl breakpoint (Fig. ). The position of the mouse X-inactivation centre has been further delineated by the study of a panel of female Embryonic Stem (ES) cell lines carrying distal deletions of varying size on one of the X chromosomes(''). Only the HD3 cell line and two other cell lines carrying deletion breakpoints distal to HD3 demonstrated X-inactivation indicating that an X-inactivation centre lies proximal to the HD3 breakpoint. Both the T16H breakpoint and the HD3 breakpoint have been localiscd to band D on the mouse X chromosome, thus apparently localising