IκB-α inhibits transcription factor NF-κB by retaining it in the cytoplasm. Various stimuli, typically those associated with stress or pathogens, rapidly inactivate IκB-α. This liberates NF-κB to translocate to the nucleus and initiate transcription of genes important for the defense of the organism. Activation of NF-κB correlates with phosphorylation of IκB-α and requires the proteolysis of this inhibitor. When either serine-32 or serine-36 of IκB-α was mutated, the protein did not undergo signal-induced phosphorylation or degradation, and NF-κB could not be activated. These results suggest that phosphorylation at one or both of these residues is critical for activation of NF-κB.