IkB-E (I-kappa-B-epsilon), also known as NFKBIE (NFkB inhibitor epsilon) or IKBE, is a 500 amino acid protein that belongs to the NFkB inhibitor family. Localizing to the cytoplasm, IkB-E is highly expressed in spleen, testis and lung, with lower levels of expression found in kidney, pancreas, heart, placenta, brain, granulocytes and macrophages. IkB-E inhibits c-Rel and NFkB subunits p50, p52 and p65 by forming a complex with them in the cytoplasm, preventing them from activating genes in the nucleus. IkB-E undergoes serine phosphorylation, resulting in the protein being marked for destruction via the ubiquitination pathway.