During early mammalian embryogenesis, one of the two X chromosomes in somatic 
cells of the female becomes inactivated through a process that is thought to 
depend on a unique initiator region, the X-chromosome inactivation center (Xic). 
The recently characterized Xist sequence (X-inactive-specific transcript) is 
thought to be a possible candidate for Xic. In mice a further genetic element, 
the X chromosome-controlling element (Xce), is also known to influence the 
choice of which of the two X chromosomes is inactivated. We report that a region 
of the mouse X chromosome lying 15 kb distal to Xist contains several sites that 
show hypermethylation specifically associated with the active X chromosome. 
Analysis of this region in various Xce strains has revealed a correlation 
between the strength of the Xce allele carried and the methylation status of 
this region. We propose that such a region could be involved in the initial 
stages of the inactivation process and in particular in the choice of which of 
the two X chromosomes present in a female cell will be inactivated.