We have characterized a flat cellular variant of HTLV-1 Tax-transformed rat fibroblasts, 5R, which is unresponsive to all tested NF-κB activating stimuli, and we report here its genetic complementation. The recovered full-length cDNA encodes a 48 kDa protein, NEMO (N F-κB E ssential MO dulator), which contains a putative leucine zipper motif. This protein is absent from 5R cells, is part of the high molecular weight IκB kinase complex, and is required for its formation. In vitro, NEMO can homodimerize and directly interacts with IKK-2. The NEMO cDNA was also able to complement another NF-κB–unresponsive cell line, 1.3E2, in which the protein is also absent, allowing us to demonstrate that this factor is required not only for Tax but also for LPS, PMA, and IL-1 stimulation of NF-κB activity.