The androgen receptor (AR) mediating androgen effects is a member of the steroid and thyroid hormone receptor gene superfamily encoding ligand-dependent nuclear transcription factors. They can be divided into three main domains, an N-terminal domain which modulates transcription efficiency, a central DNA domain which binds to a target gene hormone response element, and a C-terminal hormone binding domain. Androgens are necessary for normal male development and function. They exert their effects on target tissue through binding to the AR, followed by association of the AR complex with specific binding sites on DNA. This AR complex may induce or repress gene transcription. Mutations in the AR gene are associated with androgen insensitivity syndrome, a disorder that causes XY genotypic males to develop as phenotypic females because of their inability to respond to androgens. AR mutations have also been identified in human prostate cancer specimens and the LNCaP human prostate cancer cell line. Monoclonal and polyclonal antibodies to the AR have identified AR-positive cells in a variety of tissues including male and female sexual organs, kidney, liver, adrenal cortex, pituitary gland, skeletal, cardiac, and smooth muscle cells. The androgen receptor migrates at a molecular weight of ~100 kD in SDS-PAGE. G122–25 recognizes an epitope localized within the DNA-binding domain (between amino acids 486 and 651) of the human androgen receptor. Additionally, G122–25 has been shown to recognize androgen receptors in frozen rat prostate tissue sections. G122–25 does not cross-react with estrogen or progesterone receptors. Purified recombinant human androgen receptor protein was used as immunogen.