Mammalian dosage compensation requires silencing of one of the two X chromosomes 
in females and is controlled by the X inactivation center (Xic). Xic contains 
many of the regulatory elements for the mutual interplay of X-inactive specific 
transcript (Xist) and Tsix, the antisense counterpart of Xist. The regulatory 
elements control X chromosome inactivation (XCI) via the formation of DNA-DNA 
and DNA-protein complexes with cis- and trans-acting factors. However, the 
process-dependent regulation of Xist/Tsix by these elements in each XCI process 
remains largely unknown. In this study, a 6-thioguanine-resistant female F(1) 
hybrid mouse cell line (designated HOBMSKI2) was constructed from a cross 
between a female HPRT-deficient transgenic mouse (designated BM3) and a male 
wild type Mus spretus mouse (designated MS), which enabled the direct 
discrimination of both allele-specific expression of X-linked genes and 
allele-specific binding of proteins associated with XCI due to DNA polymorphisms 
between BM3 and MS. Using this cell line, we found that Tsix on the active X 
chromosome (Xa) was not expressed in somatic cells despite the fact that CTCF, 
which ensures Tsix expression in embryonic stem cells, was still bound to the 5' 
end of Tsix on Xa, implying that CTCF may function differently during each XCI 
process and its trans-activating activity for Tsix expression may be lost in the 
maintenance process. In addition, the monoallelic expression of Tsix on Xa was 
inhibited by epigenetic modification of the chromatin in the maintenance 
process, which was mediated by protein complexes recruited by MeCP2. The results 
indicate the value of HOBMSKI2 in directly detecting the allele-specific binding 
of CTCF and MeCP2 to the 5' end of Tsix. The HOBMSKI2 mouse line is a versatile 
and useful resource for studying the molecular mechanism of the XCI process.