NIK (NF-kB-inducing kinase) is a member of the MAP kinase kinase kinase family that binds TRAF2 and stimulates NF-kB activity. NIK was initially isolated from a human B cell cDNA library and contains 795 amino acids with an apparent molecular weight of slightly more than 97 kDa on SDS gel. NIK is a serine/threonine kinase and its kinase activity contributes to IkB phosphorylation. The carboxyl terminal segment of NIK binds TRAF2. A mutant NIK with intact carboxyl terminus but without the two lysine residues at its catalytic domain serves as a dominant-negative inhibitor for NF-kB activation. NIK also interacts with TRAF6 and mediates IL-1-induced NF-kB activation.