WASHINGTON (AP) — Women are far more likely than men to get autoimmune diseases, when an out-of-whack immune system attacks their own bodies — and new research may finally explain why. One theory is that the X chromosome might be a culprit. The X chromosome is packed with hundreds of genes, far more than males’ much smaller Y chromosome. Every female cell must switch off one of its X chromosome copies, to avoid getting a toxic double dose of all those genes. “We think that’s really important, for Xist RNA to leak out of the cell to where the immune system gets to see it.

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