Human RelA-associated inhibitor (RAI) comprises 351 amino acids and is capable of binding to p65, a subunit of the transcription factor NF.kappa.B. A process for producing RAI, a cDNA encoding it, a fragment capable of hybridizing selectively to the cDNA sequence, a plasmid for expression and duplication comprising the cDNA, a host cell transformed with the plasmid, an antibody against the polypeptide, a pharmaceutical composition comprising the polypeptide or the antibody against it and a method for diagnosis of diseases using the antibody against the RAI are disclosed.