Topic: Counting Cells That Ensure Gene Balance

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Two are one too many – this is the motto used by cells of a female organism: These contain two X chromosomes, one of which always becomes inactivated. How does the cell recognize that it contains two of these sex chromosomes and how does it choose which one to turn off? Scientists of the German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ), working together with French colleagues, have now been able to elucidate an early step in this complex process.

[TD bgColor=#336699][FONT color=#ffffff][STRONG][EM]Female embryonic stem cells differentiated for 2 days. Red: X-chromosomes, green: X-inactivation centers (Xics)[/EM][/STRONG][/FONT]

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[P]Forty-five years ago, British scientist Mary Lyon already described this chromosome i. . .