E47 is an immunoglobulin enhancer DNA-binding protein that contains a basic region-helix-loop-helix (b/HLH) domain. This structural motif defines a class of transcription factors that are central to the developmental regulation of many tissues. Its function is to provide a dimerization interface through the formation of a parallel four-helix bundle, resulting in the juxtaposition of two basic DNA-recognition α-helices that control sequence-specific DNA-binding. In order to gain insight into the biophysical nature of b/HLH domains, we have initiated structural studies of the E47 homodimer by NMR. Sequence-specific resonance assignments have been obtained using a combination of heteronuclear double- and triple-resonance NMR experiments. The secondary structure was deduced from characteristic patterns of NOEs, 13Cα/β chemical shifts, and measurements of 3JHNHα scalar couplings. Except for the basic region recognition helix, the secondary structural elements of the E47 homodimer are preserved in the absence DNA when compared with the co-crystal structure of E47 bound to DNA (Ellenberger T, Fass D, Arnaud M, Harrison SC, 1994, Genes & Dev 8:970–980). As expected, the DNA-binding helix is largely unstructured, but does show evidence of nascent helix formation. The HLH region of E47 is structured, but highly dynamic as judged by the rapid exchange of backbone hydrogen atoms and the relatively weak intensities of many of the NOEs defining the dimerization helices. This dynamic nature may be relevant to the ability of E47 both to homodimerize and to heterodimerize with MyoD, Id, and Tal1.