We isolated a novel gene termed interleukin (IL)-1-inducible nuclear ankyrin-repeat protein (INAP), of which expression was specifically induced by IL-1 in OP9 stromal cells. The INAP has ankyrin-repeat motifs and shares weak amino acid sequence homology with Bcl-3 and other IkappaB family members. The human genomic INAP gene found in the NCBI data base is located at chromosome 3q3.11. Northern blot analyses revealed that INAP was not expressed in any examined tissues without stimulation, but INAP expression was rapidly and transiently induced by IL-1 although not by tumor necrosis factor alpha nor by phorbol 12-myristate 13-acetate in OP9 cells. Immunoblots with anti-INAP-specific antibody demonstrated that INAP was rapidly and specifically produced by IL-1 stimulation and was predominantly localized in the nucleus. Immunofluorescence stainings showed that the INAP newly synthesized by IL-1 stimulation was promptly translocated into the nucleus, and FLAG-tagged INAP forcibly expressed in NIH/3T3 cells was also specifically localized in the nucleus. The possible interaction of INAP with RelA/p65, NF-kappaB1/p50, NF-kappaB2/p52, C/EBPbeta, and retinoid X receptor was examined, but we could detect none of these interactions in the nuclear extracts of IL-1-stimulated cells. Unlike Bcl-3 and other IkappaB family members, INAP may play a unique role in IL-1-induced specific gene expression and/or signal transduction in the nucleus.