The transcription factor GA-binding protein (GABP) is composed of two subunits, the Ets-related GABP-alpha and a GABP-alpha-associated subunit, GABP beta. GABP alpha binds to a specific DNA sequence and GABP beta exists as b1 and b2 splice variants that differ in their C-termini. In primary neuronal cultures, GABP beta is expressed in both the cytoplasm and the nucleus, whereas GABP alpha is expressed mainly in the nucleus. GABP is constitutively expressed as either a GABP alpha beta heterodimer or a GABP alpha b heterotetramer, both of which can modify GABP-dependent transcription in vitro and in vivo . The GABP alpha beta tetrameric complex performs many different functions, such as stimulating transcription of the adenovirus E4 gene, differentially activating BRCA1 expression in human breast cell lines, potentiating Tat-mediated activation of long terminal repeat promoter transcription and viral replication in certain cell types, acting as a coordinator of mitochrondrial and nuclear transcription for cytochrome oxidase in neurons and assisting in the regulation of rpL32 gene transcription.