The androgen receptor belongs to the family of nuclear receptors and contains three functional domains: a carboxy-terminal hormone binding region, a central cystein rich DNA binding region and an amino-terminal region involved in the expression of androgen regulated genes. Cloning of the complementary DNA encoding the androgen receptor enabled the characterization of the molecular defects associated with androgen insensitivity syndromes, X-linked disorders resulting from androgen action defects in target cells. Moreover, androgen receptor gene alterations have been recently described in two unrelated diseases: male breast cancer and spinal and bulbar muscular atrophy. Our group have identified 16 androgen receptor gene alterations in patients with androgen insensitivity syndrome, an amino acid substitution in a patient with a partial androgen insensitivity syndrome and a breast cancer. In 2 families, the molecular diagnosis of spinal and bulbar muscular atrophy has been performed.