AR (androgen receptor) is a member of the steroid superfamily of ligand-dependent transcription factors. They regulate eukaryotic gene expression and affect cellular proliferation and differentiation in target tissues. AR consists of an amino terminal modulating domain, a central DNA-binding domain, a hinge region and a carboxyl terminal ligand binding domain. Defects in AR are the cause of androgen insensibility syndrome (genital disorders), X-linked spinal and bulbar muscular atrophy (Kennedy disease), metastatic prostate cancer and infertility male syndrome.