The inactive X chromosome (green) visits the nucleolus (red) to maintain its inactivity.

LEE/ELSEVIER

In female mammalian cells, switching off the second X chromosome requires coating it in noncoding Xist RNA and converting it into heterochromatin. Li-Feng Zhang, Khanh Huynh, and Jeannie Lee (HHMI, Boston, MA) now suggest that this inactive state is maintained in each cell division by a visit to the periphery of the nucleolus.

A study in 1949 revealed that the inactive X (Xi) appeared as a nucleolar satellite in cat neurons. Despite this finding, the overwhelming impression in more recent decades has been that, in most cell types, the Xi is associated with the periphery of the nucleus, not nucleolus. The nuclear periphery, however, is also the preferred location for the active X (Xa). Using mouse embryonic stem cells, Zhang and colleagues now show that, although both Xs are often seen...

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