Gel filtration experiments using extracts from IFN-treated human HeLa and Daudi cells showed that 2',5'-oligoadenylate (2-5A) synthetase (OAS2, p69) exists as a dimer of 160 kDa (Marie I et al. 1990). Biochemical and mutational studies demonstrated that dimerization of OAS2 protein is required for its enzyme activity (Sarkar SN et al. 1999). Further, photo affinity cross-linking and peptide mapping to study the substrate binding sites in OAS2 have suggested that OAS2 is active only as a dimer because it catalyzes the joining of the acceptor substrate bound to one subunit to the donor substrate bound to the other subunit (Sarkar SN et al. 2002).