NIK (NF-. B-inducing kinase) is a member of the MAP kinase kinase kinase family that binds TRAF2 and stimulates NF-. B activity. NIK, initially isolated from a human B cell cDNA library, 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 I. B 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-. B activation. NIK also interacts with TRAF6 and mediates IL-1-induced NF-. B activation.