The present invention provides an isolated nucleic acid molecules encoding I.kappa.B kinase (IKK) catalytic subunit polypeptides, which are associated with an IKK serine protein kinase that phosphorylates a protein (I.kappa.B) that inhibits the activity of the NF-.kappa.B transcription factor, vectors comprising such nucleic acid molecules and host cells containing such vectors. In addition, the invention provides nucleotide sequences that can bind to a nucleic acid molecule of the invention, such nucleotide sequences being useful as probes or as antisense molecules. The invention also provides isolated IKK catalytic subunits, which can phosphorylate an I.kappa.B protein, and peptide portions of such IKK subunit. In addition, the invention provides anti-IKK antibodies, which specifically bind to an IKK complex or an IKK catalytic subunit, and IKK-binding fragments of such antibodies. The invention further provides methods of substantially purifying an IKK complex, methods of identifying an agent that can alter the association of an IKK complex or an IKK catalytic subunit with a second protein, and methods of identifying proteins that can interact with an IKK complex or an IKK catalytic subunit.