Date Published: July 13, 2004
Publisher: Public Library of Science
Partial Text: In mammals, males usually have one X and one Y chromosome and females have two X chromosomes. This crucial difference sets the sexes apart, but also creates a problem—female cells have the potential to turn out twice as much X-based gene product as necessary. Males with multiple–X chromosome syndromes face a similar problem. As if to avoid an overdose of X-related proteins, cells in the early embryo inactivate all but one X chromosome. The choice of which X (or Xs) to inactivate is apparently random, but once made, it persists across cell divisions and the specializations that determine a cell’s ultimate fate, or type.