Etiologic classification of severe hypospadias: implications for prognosis and management

Identification of the Rare Ala871Glu Mutation in the Androgen Receptor Gene Leading to Complete Androgen Insensitivity Syndrome in an Adolescent Girl with Primary Amenorrhea

Complete Androgen Insensitivity Syndrome (CAIS) is a rare genetic condition by mutations in the androgen receptor (AR) gene resulting in target issue resistance to androgens and a female phenotype in genetically male individuals. A 16-year-old phenotypically female individual presented to our clinic with primary amenorrhea. Her clinical evaluation showed normal female external genitalia, Tanner III breast development and sparse pubic and axillary hair (Tanner stage II). Hormonal assessment revealed increased concentrations of Luteinizing Hormone (LH), Testosterone and Antimüllerian Hormone (AMH). Image studies detected no uterus or gonads, but a blind vagina and the karyotype was 46, XY. These findings suggested the diagnosis of CAIS, and genetic testing of the AR gene revealed a rare pathogenic mutation of cytosine to adenine (c.2612C>A) replacing alanine with glutamic acid at position 871 (p.Ala871Glu) in the AR, previously described once in two adult sisters. The patient underwent gonadectomy and received hormonal replacement therapy. This study expands the AR mutation database and shows the complexity and the importance of prompt diagnosis, proper management, and follow-up for CAIS patients, underlining the need for standardized protocols.

Prenatal Diagnosis and Classification of Fetal Hypospadias: The Role and Value of Magnetic Resonance Imaging

BACKGROUND

Prenatal diagnosis and classification of hypospadias are difficult and of value for management during perinatal and neonatal periods. The conventional approach for prenatal diagnosis of hypospadias is ultrasound; however, this technique may be inconclusive in certain cases, which prompts for further exploration with magnetic resonance imaging (MRI).

PURPOSE

To investigate the role of MRI in the prenatal diagnosis and classification of fetuses with hypospadias.

STUDY TYPE

Retrospective.

POPULATION

Thirty-five fetuses (median gestational age = 37, range 24-39 weeks) with possible hypospadias.

FIELD STRENGTH/SEQUENCE

Single-shot fast spin echo T2-weighted imaging, fast imaging employing steady-state acquisition (FIESTA), and three-dimensional FIESTA acquired at 1.5 T.

ASSESSMENT

Diagnosis and classification of hypospadias using MRI were performed by three experienced radiologists based on MRI features, including a short penile shaft, abnormal penile tip, penile curvature, bifid scrotum, "tulip sign," and penoscrotal transposition. The accuracy of MRI in the diagnosis and classification of hypospadias was assessed in comparison to postnatal clinical diagnosis. The interobserver agreement between radiologists was also assessed.

STATISTICAL TESTS

Kendall's W test was applied to assess the interobserver agreement between radiologists. Taking postnatal clinical diagnosis as the reference standard, the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were calculated.

RESULTS

Of the 35 fetuses, 24 cases were confirmed as hypospadias through postnatal clinical diagnosis. The interobserver agreement between radiologists was substantial (Kendall's W = 0.781, P < 0.001). Of the 24 confirmed cases (13 cases of severe hypospadias and 11 cases of mild hypospadias), 22 cases were correctly diagnosed by MRI. The accuracy of MRI in the diagnosis of hypospadias, severe hypospadias, and mild hypospadias was 85.71%, 82.86%, and 80.00%, respectively.

DATA CONCLUSION

MRI has good performance in the diagnosis of fetal hypospadias. In addition, MRI could help evaluate the severity of fetal hypospadias.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY: Stage 2.

Molecular diagnostics of disorders of sexual development: an Indian survey and systems biology perspective

We aimed to survey the monogenic causes of disorders of sex development (DSD) and thereby its prevalence in India. This study revealed mutations resulting in androgen insensitivity syndrome, 5α-reductase type 2 deficiency, and gonadal dysgenesis were commonly reported. Intriguingly, AR deficits were the most prevalent (32 mutations) and of 11/26 missense mutations were in exons 4-8 (encoding ligand binding domain). The unique features of SRD5A2 defects were p.R246Q (most prevalent) and p.G196S could be mutational hotspots, dual gene defects (p.A596T in AR and p.G196S in SRD5A2) in a patient with hypospadias and novel 8 nucleotide deletion (exon 1) found in a patient with perineal hypospadias. Deficits in SRY, WT1, DHH, NR5A1, and DMRT1 caused 46,XY gonadal dysgenesis. Notably, mutations in AR, SRD5A2, MAMLD1, WT1, and MAP3K1 have led to hypospadias and only one CYP19A1 mutation caused aromatase deficiency was reported to date. Data mining from various databases has not only reinforced the role of well-established genes (e.g., SRY, WT1, DHH, NR5A1, DMRT1, AR, SRD5A2, MAMLD1) involved in DSD but also provided us 12 more potential candidate genes (ACVR1, AMHR2, CTNNB1, CYP11A1, CYP19A1, FGFR2, FGF9, PRKACA, PRKACG, SMAD9, TERT, ZFPM2), which benefit from a close association with the well-established genes involved in DSD and might be useful to screen owing to their direct gene-phenotype relationship or through direct functional interaction. As more genes have been revealed in relation to DSD, we believe ultimately it holds a better scenario for therapeutic regimen. Despite the advances in translational medicine, hospitals are yet to adopt genetic testing and counseling facilities in India that shall have potential impact on clinical diagnosis. Abbreviations: 5α-RD2: 5α-Reductase type 2; AIS: androgen insensitivity syndrome; AMH: antimullerian hormone; AMHR: antimullerian hormone receptor; AR: androgen receptor gene; CAH: congenital adrenal hyperplasia; CAIS: complete AIS; CAH: congenital adrenal hyperplasia; CHH: congenital hypogonadotropic hypogonadism; CXORF6: chromosome X open reading frame 6 gene; CYP19A1: cytochrome P450 family 19 subfamily A member 1 gene; DHT: dihydrotestosterone; DMRT1: double sex and mab-3 related transcription factor 1 gene; DSD: disorders of sexual development; GD: gonadal dysgenesis; HGMD: human gene mutation database; IH: isolated hypospadias; MAMLD1: mastermind like domain containing 1 gene; MIS: mullerian inhibiting substance; NTD: N-terminal domain; OT DSD: ovotesticular DSD; PAIS: partial AIS; SOX9: SRY-related HMG-box 9 gene; SRY: sex-determining region Y gene; STAR: steroidogenic acute regulatory protein gene; SRD5A2: steroid 5 alpha-reductase 2 gene; T DSD: testicular DSD; T: testosterone; WNT4: Wnt family member 4 gene; WT1: Wilms tumor 1 gene; Δ₄: androstenedione.

Use of Hormones, Tissue Factors and Bioengineering in the Management of Hypospadias

Hypospadiology is a rapidly evolving field. Progress in the understanding of how hormonal therapy affects the growth of the phallus has allowed surgeons to optimize the tissues for surgery. But conflicting data from a number of studies and a lack of consensus on drugs, their dosing, mode of delivery and timing of use means that the creation of protocols is unlikely to happen in the near future. Nonetheless, there is a hope and the standardization of scientific reporting will make it easier to compare data at the global level. There are reports of the increasing incidence of hypospadias and the etiology is thought to be multifactorial. Although complex interactions between genetic polymorphisms and the environment make it difficult to identify the exact factors responsible for hypospadias, the advent of massively parallel gene sequencing, large scale epigenetic screens and CRISPR technology will definitely ease the process. The knowledge of culprit genes will not only broaden our understanding of embryology and growth but will also enable us to predict and/or modify tissue healing. Advances in tissue engineering are also expected to provide a plethora of biomaterials for urethral reconstruction. The development of this field is directly linked with the elucidation of the processes of proliferation and vascularization coupled with the cataloguing of the growth factors involved. One can safely conclude that the exciting new advances in the field will have far reaching consequences on patient care and counselling.

Aetiology of hypospadias: a systematic review of genes and environment

BACKGROUND

Hypospadias is a common congenital malformation of the male external genitalia. Most cases have an unknown aetiology, which is probably a mix of monogenic and multifactorial forms, implicating both genes and environmental factors. This review summarizes current knowledge about the aetiology of hypospadias.

METHODS

Pubmed was used to identify studies on hypospadias aetiology published between January 1995 and February 2011. Reference lists of the selected manuscripts were also searched to identify additional studies, including those published before 1995.

RESULTS

The search provided 922 articles and 169 articles were selected for this review. Studies screening groups of patients with hypospadias for single gene defects found mutations in WT1, SF1, BMP4, BMP7, HOXA4, HOXB6, FGF8, FGFR2, AR, HSD3B2, SRD5A2, ATF3, MAMLD1, MID1 and BNC2. However, most investigators are convinced that single mutations do not cause the majority of isolated hypospadias cases. Indeed, associations were found with polymorphisms in FGF8, FGFR2, AR, HSD17B3, SRD5A2, ESR1, ESR2, ATF3, MAMLD1, DGKK, MID1, CYP1A1, GSTM1 and GSTT1. In addition, gene expression studies indentified CTGF, CYR61 and EGF as candidate genes. Environmental factors consistently implicated in hypospadias are low birthweight, maternal hypertension and pre-eclampsia, suggesting that placental insufficiency may play an important role in hypospadias aetiology. Exogenous endocrine-disrupting chemicals have the potential to induce hypospadias but it is unclear whether human exposure is high enough to exert this effect. Other environmental factors have also been associated with hypospadias but, for most, the results are inconsistent.

CONCLUSIONS

Although a number of contributors to the aetiology of hypospadias have been identified, the majority of risk factors remain unknown.

Isolated micropenis reveals partial androgen insensitivity syndrome confirmed by molecular analysis

Partial androgen insensitivity syndrome (PAIS) is the milder variant of androgen receptor (AR) defects. The subtle effects of AR mutations present in a patient with micropenis, peno-scrotal hypospadias, infertility, clitoromegaly and posterior labial fusion. We studied the association of isolated micropenis with the genetic defects resulting in androgen resistance, that is, AR gene defects and 5-alpha reductase type 2 (SRD5A2) deficiency. We describe two cases of isolated micropenis: one in a 14-year-old boy and the other in a 3-year-old boy who was followed until he was 10 years old. There were no findings of hypospadias, cryptorchidism or gynecomastia in either of these patients. Serum gonadotrophin and androgen levels were obtained and karyotyping was done. Human chorionic gonadotropin (hCG) stimulation testing assessed the functional capacity of the testes. DNA was extracted from peripheral leukocytes, and all exons of the SRD5A2 and AR genes were amplified by polymerase chain reaction and sequenced. In both patients, baseline testosterone (T) level was low and the values were elevated after hCG testing. The sequence of the SRD5A2 gene was normal in patient 1, and a heterozygous polymorphism, V89L, was found in patient 2. Two known mutations, P390S and A870V, were identified in patients 1 and 2, respectively. Mutations in the AR gene can be associated with isolated micropenis without other features of PAIS, such as hypospadias or gynecomastia. This underlines the importance of including AR gene analysis in the evaluation of isolated micropenis with normal plasma T to ensure proper management of the patient and appropriate genetic counseling for the family.

Detection and incidence of anomalies associated with hypospadias

Hypospadias has been associated with synchronous congenital anomalies, especially in the urogenital system, and routine screening of patients with hypospadias has been advocated. In recent years, ultrasound (US) has replaced intravenous pyelography (IVP) as the primary screening test for urological deformities, yet there has never been a study of the relative diagnostic efficacy of the two tests in these patients. In this study, we assessed the incidence of urogenital and extraurogenital congenital anomalies in our hypospadias patients that were noted during physical examination and/or laboratory and imaging screening, and evaluated the efficacy of our changing routine screening protocols. We conducted a retrospective analysis of the charts of all hypospadias patients seen at Assaf Harofeh Medical Center. One hundred and sixty-three hypospadias patients fulfilled the documentation criteria we set for this study. We found a high incidence of urogenital and extraurogenital anomalies associated with hypospadias and noted a previously unreported increased incidence of various forms of facial dysmorphism. Additionally, we noted that the accepted concept that US or IVP alone were satisfactory screening tools for asymptomatic low-grade hypospadias patients is probably incorrect. Based on our findings, we recommend that initial screening of all asymptomatic hypospadias patients include US, cystogram, urinalysis, and urine culture.

Hypospadias: an update

Hypospadias is the most common congenital anomaly of the penis. The problem usually develops sporadically and without an obvious underlying cause. The ectopically positioned urethral meatus lies proximal to the normal site and on the ventral aspect of the penis, and in severe cases opens onto the scrotum or perineum. The foreskin on the ventral surface is deficient, while that on the dorsal surface is abundant, giving the appearance of a dorsal hood. Chordee is more common in severe cases. Cryptorchidism and inguinal hernia are the most common associated anomalies. The frequency of associated anomalies increases with the severity of hypospadias. For isolated anterior or middle hypospadias, laboratory studies are not usually necessary. Screening for urinary tract anomalies should be considered in patients with posterior hypospadias and in those with an anomaly of at least one additional organ system. The ideal age for surgical repair in a healthy child is between 6 and 12 months of age. Most cases can be repaired in a single operation and on an outpatient basis. Even patients with a less than perfect surgical result are usually able to enjoy a satisfactory sexual life.

The development of the male genitourinary system: III. The formation of the spongiose and glandar urethra

It is generally agreed that the urethral plate disintegrates, resulting in the urethral groove. This is subsequently transformed into the urethra by fusion of the urethral folds, which flank its sides. Recently, the existence of such a groove and folds has been denied and this challenge to the long accepted existence of such folds is significant since hypospadias is considered to result from failure of their fusion. The present studies indicate that mesodermal fold formation and its subsequent subepithelial fusion across the midline plays an essential role in urethral tube formation. Disruption of this process readily explains common congenital abnormalities of the urethra.

Absence of Y chromosome microdeletions in patients with cryptorchidism and hypospadias

Microdeletions of the Y chromosome have been observed in some patients with cryptorchidism and severe defects of spermatogenesis. We investigated whether microdeletions of the Y chromosome may be present in patients with cryptorchidism and hypospadias. Peripheral blood was obtained from 20 male patients 5.8 +/- 4.1 years (range: 0.4-14 years) with cryptorchidism and hypospadias for somatic DNA analysis of Y chromosome using multiplex polymerase chain reaction. These patients had no identifiable genetic syndrome, other genitourinary malformations or an abnormal karyotype. We evaluated the presence or absence of amplification using a set of 34 different sequence-tagged sites (STS) in each patient. All patients showed normal length amplifications for each of the regions evaluated, suggesting that microdeletions of the Y chromosome are not a frequent cause of hypospadias associated with cryptorchidism.

Anti-Müllerian hormone levels in serum from human foetuses and children: pattern and clinical interest

Serum anti-Müllerian hormone (AMH) determination has been used to investigate gonadal development and abnormal sexual differentiation, but until recently, it was based on assays developed by specialized laboratories. A short time ago, a sensitive assay kit was developed commercially (Immunotech-Beckman Coulter) for clinical use. With this method, we established usual levels of serum AMH in fetuses, newborns, and pre-pubertal children, and evaluated the clinical value of this assay. AMH measurement required only 25 microl of sample and could be performed within 3 h. In females, AMH emerged after birth at low levels (median: 4 ng/ml). In males, AMH levels remained stable during fetal life (median: 44.4 ng/ml), peaked in the first months of life to reach a median of 124.7 ng/ml, then fell with wide individual variations. Cord blood AMH levels at birth may be useful to investigate ambiguous genitalia suspected prenatally. In children with isolated microphallus or hypospadias, decreased AMH values are in favor of testis dysfunction. When testes cannot be palpated, a single determination of serum AMH levels can distinguish between anorchia and cryptorchidism.

Mutation of the androgen receptor at amino acid 708 (Gly-->Ala) abolishes partial agonist activity of steroidal antiandrogens

Mutation of a single amino acid in the ligand-binding domain (LBD) of the human androgen receptor (hAR) can induce functional abnormalities in androgen binding, stabilization of active conformation, or interaction with coactivators. The Gly708Ala and Gly708Val substitutions are associated with partial and complete androgen insensitivity syndromes, respectively. In this work, we introduced Ala, Val, and aromatic Phe mutations at position 708 on helix H3 of the hAR-LBD and tested the functional and structural consequences on hAR activity in the presence of steroidal or nonsteroidal agonists and antagonists. The residues involved in the specific recognition of these androgen ligands were identified and analyzed in the light of in vitro biological experiments and the 3D hAR-LBD structure. Our study demonstrated that the Gly708Ala mutation influenced the agonist versus antagonist activity of the ligands and confirmed the crucial role of this residue within the ligand-binding pocket (LBP) in the modulation of androgen agonists. The Gly708Ala mutation transformed the antiandrogen cyproterone acetate (CPA), a partial agonist, into a pure antiandrogen, and the pure nonsteroidal antiandrogen hydroxyflutamide in a partial agonist. From the docking studies, we suggest that CPA acts on AR through the novel mechanism called "passive antagonism".

The genetics of male undermasculinization

A review of the genetics of male undermasculinization must encompass a description of the embryology of the genital system. The dimorphism of sex development consequent upon the formation of a testis and the subsequent secretion of hormones to impose a male phenotype is highlighted. Thus, an understanding of the causes of male undermasculinization (manifest as XY sex reversal, complete and partial) includes reviewing the genetic factors which control testis determination and the production and action of testicular hormones. The study of disorders of male sex development has contributed substantially to knowledge of normal male development before birth. This knowledge has been complimented in recent years by the use of targeted murine gene disruption experiments to study the sex phenotype, although murine and human phenotypes are not always concordant. The investigation of disorders associated with male undermasculinization of prenatal onset is described briefly to complete the review.

Antimineralocorticoid 11beta-substituted spirolactones exhibit androgen receptor agonistic activity: a structure function study

In humans, spironolactone and mespirenone are well known antimineralocorticoids without C-11beta substituents. These compounds display antagonist properties by acting through the human androgen receptor (hAR). In contrast, we demonstrate here that synthetic mineralocorticoid antagonists bearing hydrophobic C-11beta substituents and C-17gamma-lactone are potent hAR agonists in vitro. The three-dimensional construction of both the ligand binding domain (LBD) of the hAR and the human mineralocorticoid receptor (hMR), based on the crystal structure of the LBD of the human progesterone receptor, revealed not only that the interactions with the steroidal A- and D-rings seemed to be crucial for stabilization of active hMR or hAR conformation, but that other steroidal substitutions could influence the agonist versus antagonist activity of ligands. The docking of synthetic compounds bearing C-11beta hydrophobic substituents within the ligand binding pocket of hAR demonstrated that precise positions of the steroid, such as C-11 and C-17, are in close contact with some residues on the receptor, C-11 with Gly 708 and C-17 with Asn705 and Thr877. These contacts are crucial for the stabilization of the active receptor conformation. Mutation of Asn705 by alanine altered the 11beta-substituted spirolactone-mediated trans-activation function of hAR, suggesting an anchoring of the C-17-lactone carbonyl group (C-22) with this residue. The stabilizing effect of the H12 helix in its active conformation is also induced by hydrophobic contacts between the Gly708 and C-11beta substituents, as recently observed with the A773G-hMR mutant in the presence of similar drugs. The study of the role of these substituents suggests efficient new directions for the drug design of selective androgen agonists.

The testosterone:androstenedione ratio in male undermasculinization

OBJECTIVE

Recent reports suggest that low testosterone:androstenedione (T:A) ratio following hCG stimulation may be a useful method of diagnosing 17beta-hydroxysteroid dehydrogenase-3 (17 betaHSD3) deficiency. The aim of this study was to establish the range of T:A ratios in cases of undermasculinization with proven aetiologies other than 17 betaHSD3.

DESIGN

Register-based study of cases of male undermasculinization reported to a central database by clinicians.

SUBJECTS

Amongst the 421 cases of under-masculinization, 114 cases had testosterone and androstenedione levels before and after hCG stimulation. Of the 114, there were 18 cases of abnormal testes, 17 cases of complete androgen insensitivity syndrome (CAIS), 68 cases of partial AIS (PAIS). Of the 17 cases of CAIS, 13 had evidence of androgen receptor (AR) dysfunction; in the PAIS cohort, 26 cases had evidence of AR dysfunction. Analysis of T:A ratios in the above cohorts and comparison of these ratios to those in a group of previously described cases of 17 betaHSD3 deficiency with a mean ratio of 0.4 (SD: 0.2).

RESULTS

The median age (range) for the CAIS, PAIS and abnormal testes cohort was 1.25 years (0.06-16.5), 0.7 years (0.02-40.3) and 0.5 years (0.04-6.5), respectively. In CAIS, the median T:A rose from 0.4 (0.1 to 8.0) to 4.5 (0.5-16.7); in PAIS, median T:A rose from 0.7 (0.1 to 15) to 3.9 (0.3-20.5); in cases with abnormal testes, median T:A rose from 0.4 (0.1 to 5.6) to 0.6 (0.1-3.6). The median post-hCG T:A ratio was significantly lower in the abnormal testes cohort (P < 0.01). None of the cases of AIS with AR mutation had a low T:A ratio. Only four out of 84 cases diagnosed as AIS had a T:A ratio less than 0.8 (mean + 2SD in 17betaHSD3 deficiency). In one of the four cases, the T:A ratio rose to 3.5 following a prolonged hCG stimulation test.

CONCLUSION

Deficiency of 17betaHSD3 should be considered in 46XY undermasculinization if the post-hCG stimulation T:A ratio is less than 0.8. However, low T:A ratios may be encountered in conditions such as abnormal testes. Before embarking on mutational analysis, we would also recommend careful evaluation for testicular dysgenesis including a prolonged hCG stimulation test in cases with a low T:A ratio.

Structural evidence for ligand specificity in the binding domain of the human androgen receptor. Implications for pathogenic gene mutations

The crystal structures of the human androgen receptor (hAR) and human progesterone receptor ligand-binding domains in complex with the same ligand metribolone (R1881) have been determined. Both three-dimensional structures show the typical nuclear receptor fold. The change of two residues in the ligand-binding pocket between the human progesterone receptor and hAR is most likely the source for the specificity of R1881 to the hAR. The structural implications of the 14 known mutations in the ligand-binding pocket of the hAR ligand-binding domains associated with either prostate cancer or the partial or complete androgen receptor insensitivity syndrome were analyzed. The effects of most of these mutants could be explained on the basis of the crystal structure.

The role of a clinical score in the assessment of ambiguous genitalia

OBJECTIVE

To improve the initial assessment of ambiguous genitalia in infants.

SUBJECTS AND METHODS

Using a specially devised scoring system, the external genitalia (external masculinization score, EMS, range 0-12) and internal reproductive structures (internal masculinization score, IMS, range 0-10) were assessed in 426 male newborns and 291 cases of ambiguous genitalia.

RESULTS

In normal male newborns, the median (10th centile) EMS was 11 (10). In the affected infants, the sex of rearing was male in 202 and female in 89 cases, respectively. The median (10-90th centile) EMS in those cases reared male, at 3.5 (2-8), was significantly higher than in cases reared as females, at 2 (1-6) (P < 0.001). The median IMS in cases reared as males and females was the same, at 10, but the scatter of values was higher for males (10-90th centile, 4-10) than for females (0-10) (P = 0.01). Infants reared as females were more likely to have a micropenis, a uterus and/or a urogenital sinus, but there were 12 cases where the sex of rearing was male despite the presence of a uterus; five infants without micropenis were reared as female and 23 with a urogenital sinus were reared as male.

CONCLUSION

The masculinization score provides a standardized format to summarize clinical features in newborn infants with ambiguous genitalia. Gender assignment does not solely depend on the appearance of the external genitalia and the nature of internal sexual organs.

Current trends in hypospadias repair

There is no single, universally applicable technique for hypospadias repair. Command of a technically straightforward repair with few complications and proven success and versatility in a reasonable range of hypospadias defects are desired goals. Several well-established techniques exist for the repair of all hypospadias defects. The Snodgrass tubularized incised plate urethroplasty, a recent contribution with exemplary early results, has become a popular technique for primary and preoperative repair of middle and anterior hypospadias. Other innovative modifications, and technical advances, such as the use of laser and tissue solder, continue to emerge. With time, these may herald improvements to even the most basic of sound principles involved in all hypospadias repair.

Hypospadias

The incidence of hypospadias is increasing. Decreased androgen sensitivity, 5alpha-reductase deficiency and chromosomal abnormalities are causes of the disease. Nonirritant suture material, coverage of the urethral reconstruction by additional tissue and perioperative care positively influence the outcome. Complication rates after repair of distal hypospadias are low. For severe hypospadias staged procedures provide good results. Buccal mucosa is promising for initial repair and repeat cases. Psychosocial function in patients with severe hypospadias is normal.

Physiology and pathophysiology of androgen action

Knowledge of the physiology of male sexual differentiation and the clinical presentation of androgen insensitivity syndromes (AIS) has led to an increasing understanding of the mechanisms of androgen action. Androgens induce their specific response via the androgen receptor (AR), which in turn regulates the transcription of androgen-responsive target genes. The androgen-dependent development of male genital structures and the induction of the normal male phenotype depends on the presence of an intact AR. Structural alterations leading to malfunction of the AR are associated with variable inhibition of virilization despite normal or even supranormal serum levels of androgens. The mapping, cloning and sequencing of the AR gene have facilitated new insights into the study of androgen action. Functional investigation of the normal and the mutant AR in vivo as well as in vitro has led to the characterization of the distinct molecular steps involved in the normal androgen action pathways that are inhibited in the androgen insensitivity syndrome.