The cytokines IL‐1 and TNF induce expression of a series of genes that regulate inflammation through activation of NF‐κB signal transduction pathways. TAK1, a MAPKKK, is critical for both IL‐1‐and TNF‐induced activation of the NF‐κB pathway. TAB2, a TAK1‐binding protein, is involved in IL‐1‐induced NF‐κB activation by physically linking TAK1 to TRAF6. However, IL‐1‐induced activation of NF‐κB is not impaired in TAB2‐deficient embryonic fibroblasts. Here we report the identification and characterization of a novel protein designated TAB3, a TAB2‐like molecule that associates with TAK1 and can activate NF‐κB similar to TAB2. Endogenous TAB3 interacts with TRAF6 and TRAF2 in an IL‐1‐and a TNF‐dependent manner, respectively. Further more, IL‐1 signaling leads to the ubiquitination of TAB2 and TAB3 through TRAF6. Cotransfection of siRNAs directed against both TAB2 and TAB3 inhibit both IL‐1 …