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The Mysterious X

Why the X chromosome got its letter isn't clear — Carl Zimmer in the New York Times wonders if it was for "ex-chromosome," "extra chromosome," or for the mathematical X meaning unknown.

In female cells, one X chromosome becomes inactivated, but whether it is the maternal or paternal one varies. Recently, Zimmer notes that researchers like Jeremy Nathans at Johns Hopkins University are exploring these changes. Nathans has turned to green and red fluorescent proteins to visualize the pattern of which X is inactivated.

"But there is also much larger-scale diversity," Nathans tells Zimmer. Whole organs can be skewed toward having one or the other X chromosome activated.

This choice to shut down the X chromosome in female cells, rather than, say, doubling up expression of the single X in male cells, may have disease implications.

Harvard Medical School's Jeannie Lee notes that when Xist, the key molecule of the inactivation process, is turned off in female mice, those mice are more likely to develop cancer. She tells Zimmer that the extra proteins from the second X chromosome may lead to uncontrollable cell division. Lee adds that effect may also be a consequence of developing stem cells from female cells as the second X chromosome may be awakened.