In this week's Science, Na Xu and her colleagues at Harvard Medical School report that mouse X chromosomes transiently pair with each other during early cell differentiation, revealing a key step in X inactivation, in which only one X remains active in females. The authors suggest that interchromosomal interactions may be a common way of regulating gene expression."For monoallelically expressed genes, (pairing) may be a more common thing than we think," said Christine Disteche of the University of Washington in Seattle, who was not involved in the study. "Maybe we haven't looked hard enough." A stretch of DNA on the mammalian X chromosome called the X-inactivation center (Xic) contains several RNA-coding genes crucial for normal X inactivation, including Xist, its antisense transcript Tsix, and Xite. Tsix and Xite mediate two key processes in the beginning of X inactivation: "counting," in which a cell...

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