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Nature Cell Biology 8, 207–209 (1 March 2006) | doi:10.1038/ncb0306-207

Sealed with a X

C|[eacute]|line Morey & Wendy Bickmore

To equalise expression of X-linked genes between males (XY) and females (XX), mammals transcriptionally silence one of the X chromosomes in female cells. Initiation of X-chromosome inactivation (XCI) involves 'counting' of the X chromosomes to ensure that only one X remains active per autosome set, and a 'choice' as to which chromosome to inactivate.