Inhibits NF-kappa-B by complexing with and trapping it in the cytoplasm. May be involved in regulation of transcriptional responses to NF-kappa-B, including cell adhesion, immune and pro-inflammatory responses, apoptosis, differentiation and growth. Controlled by sequential serine-phosphorylation, ubiquitination and degradation. Tyrosine-phosphorylation could only lead to dissociation from NF-kappa-B (By similarity). Interacts with RELA. Highly expressed in lymph node, thymus followed by liver, brain, muscle, kidney, gastrointestinal and reproductive tract. Ubiquitinated; subsequent to stimulus-dependent phosphorylation on serines, polyubiquitination targets the protein for rapid degradation via the ubiquitin system. Belongs to the NF-kappa-B inhibitor family.