> A tantalizing clue stemmed from men who have two X chromosomes and one Y chromosome, a rare condition called Klinefelter syndrome. These men run a much higher risk of suffering from autoimmune diseases, suggesting that the number of X chromosomes plays an important role.

Interesting. And apparently this is pretty common. Less than 1 in 1000 male births.

> Less than 1 in 1000 male births.

Just for clarity:

<1 in 1000 or;

1 in <1000?

I assume the latter because the former is less obvious as "pretty common" given that <1 could be something like 1e-10, but it's not clear as written.

Yes. Sorry. 1 in <1000.

Thanks for the reply! I did find my answer and forgot to include it here:

> occurring in one to two per 1,000 live births

https://en.wikipedia.org/wiki/Klinefelter_syndrome

Speaking of, here's a study showing a reduction (sort of) in autoimmunity in such persons following a course of TRT:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1905740/

A molecule responsible for deactivating women’s second X chromosome is responsible

> Women typically have two X chromosomes

Isn't this actually the definition of female?

No, there are cases where males have two X chromosomes[1][2], cases where females have more[3] or less[4] than two and of course trans-women who most commonly have XY chromosomes.

[1] https://en.wikipedia.org/wiki/Klinefelter_syndrome

[2] https://en.wikipedia.org/wiki/XXYY_syndrome

[3] https://en.wikipedia.org/wiki/Trisomy_X

[4] https://en.wikipedia.org/wiki/Turner_syndrome

> > Women typically have two X chromosomes

> Isn't this actually the definition of female?

‘Women’ and ‘female’ are not interchangeable; one is a gender characteristic, the other is a sex characteristic. The statement as made is valid: Women are statistically likely to be XX, rather than XY or XXY or no doubt many other variations. ‘XX is female’ is a convenient rubric that is easy to remember, but as is usual for such things, human bodies do not respect convenient rubrics regarding genetics and sexes.

No, they're both sex characteristics. It's just that there are individuals with sex chromosome aneuploidies who will develop a female phenotype, e.g. X0 (Turner syndrome) and XXX (trisomy X), and also XY individuals where the Y chromosome is deficient in specific ways, e.g. Swyer syndrome.

> ‘Women’ and ‘female’ are not interchangeable; one is a gender characteristic, the other is a sex characteristic.

That's not what most dictionaries say. They say a 'woman' is an adult female human.

Perhaps you meant "'XX is female' is a convenient rubric"?

Yep! Fixed, thanks.

Turner syndrome describes a biological female with just one X chromosome.

Come on man. This article is about autoimmune diseases, not witty quips about trans women.