X inactivation, the transcriptional silencing of one of the two X chromosomes in 
female mammals, achieves dosage compensation of X-linked genes relative to XY 
males. In eutherian mammals X inactivation is regulated by the X-inactive 
specific transcript (Xist), a cis-acting non-coding RNA that triggers silencing 
of the chromosome from which it is transcribed. Marsupial mammals also undergo X 
inactivation but the mechanism is relatively poorly understood. We set out to 
analyse the X chromosome in Monodelphis domestica and Didelphis virginiana, 
focusing on characterizing the interval defined by the Chic1 and Slc16a2 genes 
that in eutherians flank the Xist locus. The synteny of this region is retained 
on chicken chromosome 4 where other loci belonging to the evolutionarily ancient 
stratum of the human X chromosome, the so-called X conserved region (XCR), are 
also located. We show that in both M. domestica and D. virginiana an 
evolutionary breakpoint has separated the Chic1 and Slc16a2 loci. Detailed 
analysis of opossum genomic sequences revealed linkage of Chic1 with the Lnx3 
gene, recently proposed to be the evolutionary precursor of Xist, and Fip1, the 
evolutionary precursor of Tsx, a gene located immediately downstream of Xist in 
eutherians. We discuss these findings in relation to the evolution of Xist and X 
inactivation in mammals.