Inhibits NF-kappa-B by complexing with and trapping it in the cytoplasm. Inhibits DNA-binding of NF-kappa-B p50-p65 and p50-c-Rel complexes. Interacts with RELA, REL, NFKB1 nuclear factor NF-kappa-B p50 subunit and NFKB2 nuclear factor NF-kappa-B p52 subunit. Highly expressed in spleen, testis and lung, followed by kidney, pancreas, heart, placenta and brain. Also expressed in granulocytes and macrophages. Belongs to the NF-kappa-B inhibitor family. Note: This description may include information from UniProtKB.