Male sexual differentiation and development proceed under direct control of androgens. Androgen action is mediated by the intracellular androgen receptor, which belongs to the superfamily of ligand-dependent transcription factors. In the X-linked androgen insensitivity syndrome, defects in the androgen receptor gene have prevented the normal development of both internal and external male structures in 46, XY individuals. The complete form of androgen insensitivity syndrome is characterized by 46, XY karyotype, external female phenotype, intra-abdominal testes, absence of uterus and ovaries, blindly ending vagina, and gynecomastia. There is also a group of disorders of androgen action that result from partial impairment of androgen receptor function. Clinical indications can be abnormal sexual development of individuals with a predominant male phenotype with severe hypospadias and micropenis or of individuals with a predominantly female phenotype with cliteromegaly, ambiguous genitalia, and gynecomastia. Complete or gross deletions of the androgen receptor gene have not been frequently found in persons with the complete androgen insensitivity syndrome, whereas point mutations at several different sites in exons 2-8 encoding the DNA- and androgen-binding domain have been reported in both partial and complete forms of androgen insensitivity, with a relatively high number of mutations in two clusters in exons 5 and 7. The number of mutations in exon 1 is extremely low, and no mutations have been reported in the hinge region, located between the DNA-binding domain and the ligand-binding domain. The X-linked condition of spinal and bulbar muscle atrophy (Kennedy's disease) is characterized by a progressive motor neuron degeneration associated with signs of androgen insensitivity and infertility. The molecular cause of spinal and bulbar muscle atrophy is an expanded length (> 40 residues) of one of the polyglutamine stretches in the N-terminal domain of the androgen receptor.