Provided are proteins having NF-.kappa.B activity, which are used for diagnosing, treating or preventing diseases associated with the excessive activation or inhibition of NF-.kappa.B. Using plasmid pNF .kappa.B-Luc, cDNA encoding a protein capable of activating NF-.kappa.B has been cloned from a cDNA library constructed from human lung fibroblasts, and the DNA sequence and the deduced amino acid sequence determined. The protein, the DNA encoding the protein, a recombinant vector containing the DNA, and a transformant containing the recombinant vector are useful for screening a substance inhibiting or promoting NF-.kappa.B activation.