Inhibits NF-kappa-B by complexing with and trapping it in the cytoplasm. However, the unphosphorylated form resynthesized after cell stimulation is able to bind NF-kappa-B allowing its transport to the nucleus and protecting it to further NFKBIA-dependent inactivation. Association with inhibitor kappa B-interacting NKIRAS1 and NKIRAS2 prevent its phosphorylation rendering it more resistant to degradation, explaining its slower degradation. Interacts with THRB (via ligand-binding domain) (PubMed:7776974). Interacts with RELA and REL (By similarity). Interacts with COMMD1 (PubMed:16573520). Interacts with inhibitor kappa B-interacting Ras-like NKIRAS1 and NKIRAS2 (PubMed:10657303, PubMed:12672800, PubMed:15024091). Expressed in all tissues examined. Phosphorylated by RPS6KA1; followed by degradation. Interaction with NKIRAS1 and NKIRAS2 probably prevents phosphorylation. Belongs to the NF-kappa-B inhibitor family.