Androgen insensitivity syndrome (AIS) is a condition in which the body’s cells cannot respond normally to androgens, the group of hormones that includes testosterone. People with AIS have a 46,XY karyotype and testes that produce testosterone, but because their androgen receptors are partially or fully nonfunctional, the typical masculinizing effects of those hormones are blunted or absent. The result is a wide phenotypic spectrum, from individuals who appear entirely female at birth to those who appear male but experience infertility or subtle undermasculinization. AIS is one of the most well-studied differences of sex development, and the clinical picture is considerably more nuanced than most people realize.
How the Androgen Receptor Goes Wrong
AIS is caused by mutations in the androgen receptor (AR) gene, which sits on the X chromosome. Because individuals with a 46,XY karyotype have only one X, a single defective copy of this gene is enough to cause the condition. The mutations follow X-linked recessive inheritance, meaning a carrier mother with one working copy and one mutated copy has a fifty-fifty chance of passing the mutation to each child. Hundreds of distinct AR mutations have been catalogued. Some insert a premature stop signal, preventing the receptor protein from being built at all. Others are missense mutations that change a single amino acid, subtly altering how the receptor binds testosterone or communicates with DNA inside the cell.1The Journal of Steroid Biochemistry and Molecular Biology. Mutations of androgen receptor gene in androgen insensitivity syndromes One well-characterized example involves a single amino acid swap that creates an unwanted phosphorylation site on the receptor, which then interferes with key protein-protein interactions needed for the receptor to activate gene transcription.2Journal of Biological Chemistry. Gene Regulation Androgen Receptor Exon 1 Mutation Causes Androgen Insensitivity by Creating Phosphorylation Site and Inhibiting Melanoma Antigen-A11 Activation of NH2- and Carboxyl-terminal Interaction-dependent Transactivation
Modern sequencing has accelerated the pace of discovery. Exome sequencing studies continue to uncover novel AR variants, including frameshift mutations, splice-site disruptions, and even whole-exon deletions.3PubMed. Novel pathogenic variants in the androgen receptor gene associated with androgen insensitivity syndrome identified through exome sequencing and in silico analysis A 2024 next-generation sequencing study identified two AR variants that had never been described before, underscoring that the full mutational landscape is still being mapped.4PubMed Central. Expanding the Molecular Landscape of Androgen Insensitivity Syndrome Through Next-Generation Sequencing Despite these advances, knowing the exact mutation does not always predict how severely someone will be affected. Functional testing, such as measuring the in-vivo response of sex hormone-binding globulin to a test dose of a synthetic androgen, can add practical information about how impaired the receptor actually is in a given person.5PubMed. Functional assessment and clinical classification of androgen sensitivity in patients with mutations of the androgen receptor gene
The Spectrum From Complete to Mild
AIS is typically divided into three forms based on how much receptor function remains. Complete androgen insensitivity syndrome (CAIS) involves a total inability to respond to androgens. Individuals with CAIS are born with typical-appearing female external genitalia, develop breasts at puberty, and are raised as girls. They have testes, usually located in the abdomen or inguinal canal, but no uterus or ovaries. Menstruation never occurs. The condition is sometimes discovered only when an adolescent is evaluated for absent periods or when testes are found during hernia repair in childhood.
Partial androgen insensitivity syndrome (PAIS) covers a broad middle ground. Some degree of receptor function remains, so the external genitalia may be ambiguous at birth, ranging from predominantly female with mild clitoral enlargement to predominantly male with hypospadias or a small phallus. PAIS is sometimes called a diagnostic and therapeutic dilemma because the clinical picture varies enormously even among individuals carrying the same mutation.6PubMed Central. Partial androgen insensitivity syndrome: a diagnostic and therapeutic dilemma
Mild androgen insensitivity syndrome (MAIS) sits at the far end of the spectrum. People with MAIS appear male, go through typical male puberty, and may not learn about the condition until they seek help for infertility. Two unrelated men in one report presented with no sperm in their ejaculate and subtle signs of undermasculinization; genetic testing revealed the same AR mutation in both.7PubMed Central. Mild androgen insensitivity syndrome (MAIS): the identification of c.1783C>T mutation in two unrelated infertile men Other MAIS presentations include premature ejaculation and erectile dysfunction alongside infertility.8Bangladesh Journal of Medical Science. Mild Androgen Insensitivity Syndrome presenting in male with infertility and sexual difficulties Because these complaints are common in the general population, MAIS is almost certainly underdiagnosed. Screening for AR mutations is worth considering whenever unexplained male infertility occurs alongside even mild signs of reduced androgen action.9PubMed. Androgen insensitivity and male infertility
Puberty in Complete AIS
One of the more counterintuitive aspects of CAIS is that puberty proceeds in many ways that look typically female, even though the hormonal environment is anything but typical. The testes produce male-range or even elevated levels of testosterone, but because the cells cannot respond to it, a portion of that testosterone gets converted to estradiol through aromatization. This estradiol drives breast development and fat distribution along female lines. In one study, breast development in girls with CAIS began at a median age of about 11 years, and peak height velocity occurred around age 12, both consistent with averages for girls in the general population.10PubMed. Puberty in subjects with complete androgen insensitivity syndrome Adult height, however, tends to be taller than average for women, sitting closer to what would be expected based on male relatives.
The hormonal feedback loops behave unusually too. Luteinizing hormone (LH) levels start out in a range that overlaps with both male and female norms during early puberty but can spike dramatically later, particularly after estradiol supplementation begins.11PubMed. Gonadotropin regulation during puberty in complete androgen insensitivity syndrome with testicles in situ This hormonal quirk has implications for clinical monitoring but does not produce visible symptoms in most cases.
Gonadal Cancer Risk and the Shifting Approach to Surgery
For decades, it was standard practice to remove the testes of individuals with CAIS soon after diagnosis, sometimes in infancy, primarily out of concern for cancer. That approach has changed substantially. A systematic review that pooled data from over 450 gonadectomy and biopsy cases in people with CAIS found pre-malignant lesions in roughly six percent, with the vast majority of those occurring after puberty. Actual malignancy was found in about one percent, and every malignant case was postpubertal.12PubMed Central. Complete androgen insensitivity syndrome and risk of gonadal malignancy: systematic review Because the prepubertal cancer risk is very low, the current consensus favors delaying gonadectomy until after puberty, allowing it to occur naturally without the need for exogenous hormones.13PubMed. Timing of Gonadectomy in Patients with Complete Androgen Insensitivity Syndrome-Current Recommendations and Future Directions
Some clinicians and patients are going even further. A management protocol published in 2019 noted that in individuals with molecularly confirmed CAIS, the rate of testicular germ cell tumors may be as low as zero percent, raising the question of whether gonadectomy is necessary at all in some cases.14PubMed Central. A Management Protocol for Gonad Preservation in Patients with Androgen Insensitivity Syndrome Keeping the gonads intact has its own benefits, particularly for bone health.
Bone Health After Gonadectomy
Bone mineral density is a significant long-term concern. Women with CAIS who retain their testes tend to have bone density in the normal range, though some show reduced density in the lumbar spine. Once the gonads are removed, bone density typically drops at both the spine and hip, and hormone replacement therapy can improve but usually does not fully normalize it.15PubMed. Bone Mineral Density in Women Living with Complete Androgen Insensitivity Syndrome and Intact Testes or Removed Gonads Several factors compound the problem: the loss of androgen receptor signaling at bone itself, the timing of surgery relative to when peak bone mass would normally be achieved, gaps in hormone replacement, and inconsistent compliance with long-term therapy. For individuals who underwent prepubertal gonadectomy, early osteoporosis is a documented risk that requires proactive monitoring.16PubMed Central. Dilemmas in management of osteoporosis in patients with complete androgen insensitivity syndrome Gonad preservation into adulthood, with regular surveillance for malignancy, has emerged as one strategy to protect skeletal health.
Body Composition and Metabolic Risks
Beyond bones, the disrupted androgen signaling in CAIS appears to influence body composition and metabolic health more broadly. A study of 18 young-to-middle-aged women with CAIS found that their average body mass index was not significantly elevated overall, but the rate of obesity was notably higher than among age-matched women in the general population. Those with intact testes were more likely to be obese than those who had undergone gonadectomy. Body fat measured by imaging was higher than in both female and male comparison groups, and a substantial subset showed elevated cholesterol and markers of insulin resistance.17Sexual Development. Body Composition and Metabolic Profile in Women with Complete Androgen Insensitivity Syndrome These findings suggest that cardiovascular and metabolic screening should be part of routine long-term care, particularly as individuals age.
Brain, Cognition, and the Question of “Male” Versus “Female” Patterns
Because women with CAIS have a Y chromosome and male-typical testosterone levels but no functional androgen receptors, they represent a natural experiment for studying what drives sex differences in the brain. The findings have been striking. In the first brain imaging study to compare women with CAIS, typical women, and typical men viewing sexual images, men showed greater activation in the amygdala than either group of women. Women with CAIS and typical women responded almost identically, demonstrating that having a Y chromosome alone is not enough to produce male-typical brain responses, and that aromatization of testosterone to estradiol does not drive these particular sex differences either.18PubMed. Brain responses to sexual images in 46,XY women with complete androgen insensitivity syndrome are female-typical
A separate imaging study that used a mental rotation task, an area where men typically outperform women, found that women with CAIS showed female-typical neural activation patterns in the parietal lobe. The researchers interpreted this as evidence that sex differences in regional brain function during spatial tasks are driven by hormonal exposure rather than by genetic sex directly.19Cerebral Cortex. Neural Activation During Mental Rotation in Complete Androgen Insensitivity Syndrome: The Influence of Sex Hormones and Sex Chromosomes On cognitive testing, women with CAIS performed similarly to female controls on most tasks. One exception was emotion recognition, where women with CAIS scored closer to male controls, a pattern that researchers linked to possible X- or Y-chromosome effects distinct from hormonal ones.20PubMed. Cognitive abilities in women with complete androgen insensitivity syndrome and women with gonadal dysgenesis
Gender Identity and Psychological Well-Being
A common worry among families receiving a CAIS diagnosis is whether the child will develop a female gender identity given a 46,XY karyotype. The evidence is reassuring. In a review of 156 females with CAIS, there were no documented cases of gender change, meaning none chose to transition to a male identity.21PubMed. Gender dysphoria and gender change in androgen insensitivity or micropenis Gender dysphoria was rare. The strongest predictor of adult gender identity was the gender initially assigned at birth, a finding consistent with the broader understanding that the brain in CAIS is essentially unresponsive to androgens and develops along female-typical lines.
The picture is more complex in partial AIS. About nine percent of individuals with PAIS in the same review changed their gender, which is much higher than in CAIS though still a small minority. Those with PAIS also reported more psychological distress, with roughly a quarter experiencing some degree of sexual identity questioning.22Journal of Pediatric Urology. The challenges of androgen insensitivity syndrome Ongoing psychological support is considered an essential part of care for anyone on the AIS spectrum, but particularly for people with PAIS, where the physical presentation and social experience are less clearly defined.
Vaginal Length and Sexual Function
Most women with CAIS are born with a short vaginal pouch that ends blindly, since they have no uterus or cervix. Historically, surgical creation of a vagina (vaginoplasty) was the default treatment. The current first-line approach for many clinicians is vaginal dilation, a non-surgical technique in which graduated dilators are used to gradually lengthen the existing tissue. A large review found that vaginoplasty techniques achieved a vaginal length of seven centimeters or more in over 90 percent of cases, compared with about 75 percent for dilation alone. However, when success was measured by satisfaction with sexual activity rather than by centimeters, the gap between the two approaches disappeared.23Human Reproduction Update. An update on surgical and non-surgical treatments for vaginal hypoplasia Complication rates were significantly lower with dilation. A comparative follow-up study found that after either treatment, about a quarter of women had a vaginal length more than two standard deviations below the population average, and about half had at least one measurable sexual dysfunction. Women who had undergone vaginoplasty reported more lubrication problems than those who used dilation.24The Journal of Sexual Medicine. Long-Term Psychosexual and Anatomical Outcome After Vaginal Dilation or Vaginoplasty: A Comparative Study Neither approach is perfect, and the choice involves weighing anatomy, personal preference, and willingness to commit to the dilation regimen.
Conditions That Look Similar
Several other conditions can produce a female appearance with a 46,XY karyotype, and telling them apart matters because their management differs. Swyer syndrome involves streak gonads that produce neither testosterone nor anti-Müllerian hormone, so a uterus is present. In CAIS, the testes produce anti-Müllerian hormone normally, which causes the uterus to regress during fetal development. But distinguishing the two can be tricky: in at least one reported case, a uterus in Swyer syndrome was initially too small to visualize on imaging, leading to an incorrect diagnosis of CAIS. The uterus only became apparent after hormone replacement therapy was started.25PubMed. Differentiating Swyer syndrome and complete androgen insensitivity syndrome: a diagnostic dilemma
Another condition in the differential is 5-alpha reductase deficiency, where testosterone is produced and the receptor works but the enzyme that converts testosterone to its more potent form is impaired. A hormonal ratio test can distinguish the two conditions with high accuracy: one study found that a testosterone-to-dihydrotestosterone ratio above a specific cutoff correctly identified 5-alpha reductase deficiency with about 94 percent sensitivity and 100 percent specificity.26PubMed Central. Differences of adrenal-derived androgens in 5alpha-reductase deficiency versus androgen insensitivity syndrome Getting the diagnosis right matters enormously because 5-alpha reductase deficiency can cause significant virilization at puberty, while CAIS does not.
Fertility and Emerging Preservation Options
Fertility is one of the most emotionally difficult aspects of AIS. Women with CAIS have no uterus and produce no eggs, so biological pregnancy is not possible. At the mild end of the spectrum, the picture varies. A family study from China vividly illustrated this range: two maternal uncles with MAIS carried the same AR variant, yet one fathered two daughters naturally while the other needed assisted reproductive technology due to severely impaired sperm.27PubMed Central. Diverse phenotypes and fertility outcomes of patients with androgen insensitivity syndrome in a Chinese family harboring identical AR gene variant Even within one family sharing the same mutation, fertility outcomes can diverge sharply.
For individuals undergoing gonadectomy, some centers now offer experimental gonadal tissue cryopreservation, banking testicular tissue at the time of surgery in case future advances make it possible to derive mature gametes from stored tissue. An 11-year-old girl with PAIS underwent this procedure ahead of a planned bilateral gonadectomy.28PubMed Central. Gonadal Tissue Cryopreservation for a Girl With Partial Androgen Insensitivity Syndrome The technology is still experimental, and no one has yet produced a viable pregnancy from cryopreserved testicular tissue in this population. But the option represents a tangible shift from writing off fertility to at least preserving the possibility.
Fetal Development and Why Testes Form Normally
A question that sometimes puzzles people learning about AIS for the first time is why the testes develop at all if the androgen receptor does not work. The answer is that testis formation is driven by a different gene (SRY on the Y chromosome), not by androgens. A fetal study confirmed that in CAIS, the testes differentiate and develop normally through at least the second trimester, and anti-Müllerian hormone expression follows the same pattern as in typical male fetuses. What goes wrong is downstream: without androgen receptor function, the external genitalia do not masculinize, and the Müllerian ducts regress incompletely in some cases through mechanisms that are still not fully understood.29Fertility and Sterility. Gonadal structures in a fetus with complete androgen insensitivity syndrome and persistent Müllerian derivatives: comparison with normal fetal development
Consent, Autonomy, and Evolving Ethics
The history of AIS management is intertwined with broader debates about medical decision-making for people born with differences of sex development. For much of the twentieth century, the dominant approach emphasized early surgery, secrecy, and conformity to a binary sex assignment. Gonads were removed in infancy, patients were sometimes not told their full diagnosis, and decisions were made by physicians and parents with little input from the individuals most affected. A historical analysis of this era argued that the rise of bioethics and informed consent doctrine should have changed intersex care more than it did, but these principles collided with a medical culture focused on treating infants who were too young to participate in decisions about their own bodies.30Medical Law Review. Did Bioethics Matter? A History of Autonomy, Consent, and Intersex Genital Surgery Patient advocacy organizations have pushed hard for change, and modern guidelines increasingly emphasize deferring irreversible procedures until the individual can be involved in the decision. The shift is incomplete, but it is real.
AIS in Other Species
AIS is not unique to humans. The androgen receptor is highly conserved across mammals, and naturally occurring mutations have been documented in other species, providing both veterinary challenges and research insights. In horses, two cases of AIS were identified with distinct AR gene variants: one a frameshift mutation that produced a drastically truncated receptor protein, the other a missense change in the androgen-binding domain.31PubMed Central. Androgen Receptor Gene Variants in New Cases of Equine Androgen Insensitivity Syndrome Affected horses present as phenotypic mares with a stallion karyotype, mirroring the human CAIS picture in broad strokes. Studying these cases across species helps researchers understand which parts of the receptor are most critical for function and which mutations are tolerated versus catastrophic.