Absence of the XIST gene from late-replicating isodicentric X chromosomes in leukaemia.

The mechanism of X-inactivation in man is thought to involve a specific cis-acting locus within the X-inactivation centre at Xq13 (1,2). The XIST gene (X inactive specific transcript) at Xq13 is ubiquitously expressed only from the inactive X and as such may be involved in or influenced by the X-inactivation process (3,4). We have localised the breakpoints on two acquired isodicentric X chromosomes associated with leukaemia to a 450 kilobase region of DNA within Xq13, which result in deletion of the XIST gene. We have demonstrated that these chromosomes remain inactive and that there is no evidence of XIST expression from the remaining intact X chromosomes. The data suggest that XIST is not required for the maintenance of X-inactivation on these somatically rearranged X chromosomes.