NF-kappaB is a key mediator for inducible transcription of various proinflammatory genes in innate immune responses, and its activity is strictly regulated by several IkappaB proteins. Although signaling pathways leading from pattern recognition receptors to NF-kappaB's activation in the cytoplasm have been studied extensively, the detail regulatory mechanisms of NF-kappaB-mediated transcriptional activity in the nucleus still remain unclear. Here we describe a unique member of the nuclear IkappaB protein family, IkappaBeta. In a gene expression analysis of dendritic cells, we found a unique gene encoding an uncharacterized protein with ankyrin repeats. As it was structurally related to the IkappaB family, the protein was named "IkappaBeta" and further characterized in the innate immune response. IkappaBeta was widely expressed in various tissues and predominantly located in the nucleus. Moreover, biochemical analysis showed that IkappaBeta associated with the p50 subunit of NF-kappaB. Knockdown of IkappaBeta by siRNA suppressed the transcription of a subset of NF-kappaB-mediated proinflammatory cytokines in LPS-stimulated and poly (I:C)-transfected macrophages. These results indicate that IkappaBeta regulates the NF-kappaB-mediated transcription of a wide variety of proinflammatory genes, playing a crucial role in the regulation of innate immune responses.