Why X Marks the Gender

With the publication this week of the gene sequence of the X chromosome, we will start to find genetic reasons for why the sexes are different, beyond XX and XY.
One clue leaps straight out of the mass of sequence data detailing the 1,098 genes found on the X: Of 40 women whose X chromosomes were looked at, all of them showed extensive variation in gene activity.
Wired News: Why X Marks the Gender