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NF-κB activator 1 (Act1), also called CIKS, is a recently identified protein with NF-κB and AP-1 activation activities through its association with the IκB kinase complex. We identified and confirmed that Act1 interacts with tumor necrosis factor receptor-associated factor 6 (TRAF6); notably, Act1 binds to TRAF6 only among TRAF family proteins. The amino-terminal half of Act1 is required for its interaction with the TRAF domain. Act1-mediated NF-κB activation was inhibited by a dominant-negative mutant of TRAF6 in a dose-dependent manner, and IL-1-induced NF-κB activation was inhibited by a high level of Act1 expression. Our results suggest that Act1 is involved in IL-1/Toll-mediated signaling through TRAF6. [Copyright &y& Elsevier]