Molecular cloning of the subunits of the transcription factor NF-kappa-B and its inhibitor I-kappa-B revealed regions of sequence homology with two different classes of proteins: the Rel/dorsal family and a heterogeneous group of proteins containing ankyrin-like repeats. Both the Rel/dorsal homology domain and the ankyrin-like repeats appear to play important roles in protein-protein interactions that regulate localization and activity of the NF-kappa-B subunits.