Tissue and lineage-specific variation in inactive X chromosome expression of the murine Smcx gene

To understand how gene expression patterns are established on the inactiveX chromosome during development, we have studied the murine gene Smcx,which is expressed from both the active and inactive mouse X chromosomes.In all tissues assayed, Smcx only partially escapes X inactivation, withexpression levels from the inactive X allele approximately 30-65% that ofthe active X allele. Additionally, inactive X expression levels differedbetween extraembryonic and embryonic tissues and among different tissuesfrom newborn and adult mice. Imprinted extraembryonic tissue had the lowestlevels of inactive X Smcx expression, whereas the highest levels were inheart. These data suggest that the chromosomal basis of X inactivationdiffers among tissues, perhaps reflecting differences in the timing orregulation of inactivation in these cell lineages.