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.