IκB [inhibitor of nuclear factor κB (NF-κB)] kinase (IKK) phosphorylates IκB inhibitory proteins, causing their degradation and activation of transcription factor NF-κB, a master activator of inflammatory responses. IKK is composed of three subunits—IKKα and IKKβ, which are highly similar protein kinases, and IKKγ, a regulatory subunit. In mammalian cells, phosphorylation of two sites at the activation loop of IKKβ was essential for activation of IKK by tumor necrosis factor and interleukin-1. Elimination of equivalent sites in IKKα, however, did not interfere with IKK activation. Thus, IKKβ, not IKKα, is the target for proinflammatory stimuli. Once activated, IKKβ autophosphorylated at a carboxyl-terminal serine cluster. Such phosphorylation decreased IKK activity and may prevent prolonged activation of the inflammatory response.