NF-κB originally identified as a B cell-specific nuclear factor binding to the intronic κ-light-chain enhancer element. It is found constitutively in the nucleus of mature B and plasma sells. In other sell types including pre-B cells, NF-κB is sequestered in the cytoplasm but can be induced by a variety of stimuli. In contrast to essentially all other mature B cells and plasma cell lines, the S107 plasmacytoma cell line lacks both constitutive and inducible κB-binding activity. A molecular characterization of the defect in these S107 cells suggests that the primary defect lies in the signal transduction pathway leading to NF-κB induction. Ectopic expression of RelB after stable transfection of these cells restores constitutive nuclear κB-binding activity. Moreover, κB-dependent transcription is also restored. Finally we demonstrate, that in contrast to parental S107 cells, the stable RelB transfectants have also regained the ability to specifically demethylate a transfected immunoglobulin κ-locus. These data suggest that RelB is critically involved in both B cell-specific transcription and demethylation directed by the intronic κ-enhancer element.