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.

It’s all about how the body handles females’ extra X chromosome, Stanford University researchers reported— a finding that could lead to better ways to detect a long list of diseases that are hard to diagnose and treat.

More than 24 million Americans, by some estimates up to 50 million, have an autoimmune disorder — diseases such as lupus, rheumatoid arthritis, multiple sclerosis and dozens more. About 4 of every 5 patients are women, a mystery that has baffled scientists for decades.

One theory is that the X chromosome might be a culprit. After all, females have two X chromosomes while males have one X and one Y.

The new research, published in the journal Cell, shows that extra X is involved — but in an unexpected way.

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