Induction of NF‐κB‐dependent transcription requires phosphorylation and subsequent degradation of I‐κB, an inhibitor of NF‐κB, followed by nuclear translocation and DNA binding of NF‐κB. Tumor necrosis factor receptor‐associated factor 2 (TRAF2) plays a role in NF‐κB activation in response to cytokines such as tumor necrosis factor α (TNFα). In this study, we purified and characterized a novel kinase (T2K, also known as TBK1 or NAK), which associates with TRAF2 and exhibits kinase activity towards I‐κBα in vitro. The physiological function of T2K was investigated using T2K‐deficient mice. Heterozygotes appear normal, but t2k−/− animals die at∼ E14. 5 of massive liver degeneration and apoptosis. Never theless, hematopoietic progenitors from T2K‐deficient fetal liver support normal lymphocyte development. Furthermore, t2k−/− embryonic fibroblasts and thymocytes do not display increased sensitivity to TNFα‐induced apoptosis. In response to either TNFα or IL‐1 induction, t2k−/− embryonic fibroblasts exhibit normal degradation of I‐κB and κB‐binding activity. However, NF‐κB‐directed transcription is dramatically reduced. These results demonstrate that, like I‐κB kinase β and the RelA subunit of NF‐κB, T2K is critical in protecting embryonic liver from apoptosis. However, T2K has a unique role in the activation of NF‐κB‐directed transcription, apparently independent of I‐κB degradation and NF‐κB DNA binding.