Gadd45 alpha, beta, and gamma proteins, also known as growth arrest and DNA damage-inducible factors, have a number of cellular functions, including cell-cycle regulation and propagation of signals produced by a variety of cellular stimuli, maintaining genomic stability and apoptosis. Furthermore, Gadd45 beta has been indicated as a major player in the endogenous NF-kappaB-mediated resistance to apoptosis in a variety of cell lines. In fibroblasts this mechanism involves the inactivation of MKK7, the upstream activator of JNK, by direct binding within the kinase ATP pocket. On the basis of a number of experimental data, the structures of Gadd45 beta and the Gadd45 beta-MKK7 complex have been predicted recently and data show that interactions are mediated by acidic loops 1 and 2, and helices 3 and 4 of Gadd45 beta. Here, we provide further evidence that Gadd45 beta is a prevailingly alpha-helical protein and that in solution it is able to form non covalent dimers but not higher-order oligomers, in contrast to what has been reported for the homologous Gadd45 alpha. We show that the contact region between the two monomers is comprised of the predicted helix 1 (residues Q17-Q33) and helix 5 (residues K131-R146) of the protein, which appear to be antiparallel and to form a large dimerisation surface not involved in MKK7 recognition. The results suggest the occurrence of a large complex containing at least an MKK7-Gadd45 beta:Gadd45 beta-MKK7 tetrameric unit whose complexity could be further increased by the dimeric nature of the isolated MKK7.