NF-κB plays a pivotal role in normal T-cell activation and may also mediate human T-cell leukemia virus (HTLV)-induced T-cell transformation. Activation of NF-κB by both T-cell costimulatory signals and the HTLV Tax protein involves stimulation of IκB kinase (IKK). As a genetic approach to dissect the intermediate steps involved in NF-κB activation in human T cells, we performed somatic cell mutagenesis to isolate signaling-defective mutant Jurkat T-cell lines. One of the mutant cell lines was shown to have a specific blockade in the IKK signaling pathway but remained competent in the c-Jun N-terminal kinase and MAP kinase pathways. Interestingly, this mutant cell line lacks expression of IKKγ, a non-catalytic component of the IKK complex. Expression of exogenous IKKγ in the mutant cells restored NF-κB activation by both the T-cell costimulation agents and Tax. These findings provide genetic evidence for the requirement of IKKγ in NF-κB signaling triggered by both T-cell costimulatory signals and HTLV-I Tax protein.