We have started the structure determination of the dimerization domain of LFB1 in solution by nuclear magnetic resonance in order to elucidate the way that the LFB1 protein dimerizes and then interacts with DNA. A 32 amino acid peptide was synthesized, and full assignment of the NMR resonances in acidic solution was achieved. The secondary structure determination is presented here. Three structurally distinct regions can be distinguished. The N-terminal region from residues 1 to 6 is extended. Two helical regions span from residues 7 to 18 and from 23 to 32. The absence of dipolar effects involving residues more than four positions apart in the sequence excludes the possibilities both of a four-helix bundle formed by two hairpins and of an antiparallel dimer; the domain must therefore be arranged as a parallel dimer formed by kinked monomers. This structural solution presents important differences from the leucine zipper-type structure observed in other transcriptional activators. Although further studies are still necessary to determine the 3D structure of the peptide, we can exclude the possibility of a coiled-coil structure.