Androgen receptor (AR) coactivator ARA70, also designated NCOA4, RFG and ELE1, is a putative co-activator that specifically enhances the activity of the androgen receptor. In human thyroid carcinomas, the Ret proto-oncogene fuses to ARA70 to form Ret/PTC3 by an intrachromosomal inversion of chromosome 10 in vivo. ARA70 is expressed as two isoforms, ARA70a and ARA70b. The shorter variant, ARA70b, results from an internal 985-bp deletion. ARA70a is widely expressed, and its expression is highest in testis and adipose tissues, whereas ARA70b is solely expressed in the testis. ARA70a can function as a ligand-enhanced co-activator of PPARg in adipocytes. ARA70 may be involved in prostate carcinogenesis and ovarian cancer and may serve as a key mediator of estrogen-androgen synergism.