Inactivation of one X chromosome occurs in XX female cells to bring about equivalence of X gene expression with male cells. It requires either a counting mechanism that triggers X inactivation in cells with more than one X or a blocking mechanism that saves a single X from an undiscerning inactivation process acting on all remaining Xs. In the August 1 Human Molecular Genetics, Laura P. O'Neill and colleagues from the Chromatin and Gene Expression Group at the University of Birmingham Medical School report that both X chromosomes are epigenetically marked in female cells, while the autosomes and the single X in male cells are unmarked, suggesting that a chromatin-based counting mechanism restricts X inactivation to cells with more than one X chromosome (Human Molecular Genetics, 12:1783-1790, August 1, 2003).

O'Neill et al. immunoprecipitated acetylated isoforms of the four core histones in mouse embryonic stem (ES) cells and...

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