Androgens are essential for male sexual differentiation and virilization and for the maintenance of reproductive functions. Their genomic effects are mediated via an intracellular androgen receptor (AR) belonging to the superfamily of nuclear receptors. Mutataions in the AR gene can lead to a broad spectrum of undervirilization from a normal male phenotype but infertility to female appearance with male chromosomal sex (partial or complete androgen insensitivity syndrome, PAIS or CAIS). However, analysis of AR gene mutations are not sufficient to explain all cases of AIS since there is no clear correlation between mutation in the AR and degree of virilization, except some rare mutations (e.g. nonsense mutations, greater deletions) which lead to CAIS. In addition the same mutation may cause different types of virilization defects and AIS can also occur in the presence of normal AR. AR gene mutations are also associated with prostate cancer, male breast carcinoma and the moto-neuro-degenerative Kennedy’s disease.