Androgen insensitivity syndrome is the result of a mutation on the X chromosome of a genetically male (XY) individual that interrupts the formation of the male sex hormone receptors. All other hormone receptors associated with sexual development are intact and functional.

Prenatally, the testes develop normally and secrete hormones (including androgens). Because the androgen receptors are not functional, the epididymis, vas deferens, seminal vesicles, and prostate do not develop. Also, the anti-mullerian hormones (whose receptors are intact) have the normal effect of preventing the development of female internal sex organs (uterus, fimbriae, Fallopian tubes, and the upper portion of the vagina).

At puberty, although the testes (still undescended because they have nowhere to descend to) secrete testosterone as normal for an XY individual, the nonfunctional receptors fail to detect it (testosterone is an androgen). As a result, these individuals develop female secondary sex characteristics. In other words, they look like women, but are genetically male. Because they do not have female internal organs, and cannot produce sperm, people with this syndrome are incapable of having children.

main source: Physiology of Behavior, 6th edition, 1998. Neil R. Carlson

One of the many terms for the condition which makes someone androgen-insensitive. Basically, the body of whomever has this (and it can be either male or female) doesn't respond to testosterone. This theoretically exists in chromosomal females, but it has very little effect in the end, and so isn't ever recognized (though they would be more busty and have a lowered libido due to the lack of any influence of testosterone, and an FTM transsexual with this condition would be unable to take advantage of hormone-replacement therapy). In a chromosomal male, this results in him (or, more accurately, her) being an androgen insensitive XY.

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