Purified 60-70%. Peptide corresponding to amino acids 658 to 674 of human IKKa (1,2), which differs from corresponding murine sequence by one amino acid. Nuclear factor kappa B (NF-kB) is a ubiquitous transcription factor and an essential mediator of gene expression during activation of immune and inflammatory responses. NF-kB mediates the expression of a great variety of genes in response to extracellular stimuli including IL-1, TNFa, and bacteria product LPS. NF-kB is associated with IkB proteins in the cell cytoplasm, which inhibit NF-kB activity. The long-sought IkB kinase (IKK), which phosphorylates IkB, and mediates IkB degradation and NF-kB activation, was recently identified by several laboratories (1-5). IKK is a serine protein kinase, and the IKK complex contains alpha and beta subunits (IKKa and IKKb). IKKa and IKKb interact with each other and both are essential for the NF-kB activation. IKKa specifically phosphorylates IkB-a. IKKa is expressed in a variety of human tissues