By using the amino acid sequence motif of tumor necrosis factor (TNF), we searched the expressed sequence tag data base and identified a novel full-length cDNA encoding 285 amino acid residues and named it THANK. THANK is a type II transmembrane protein with 15-20% overall amino acid sequence homology to TNF, LT-alpha, FasL, and LIGHT, all members of the TNF family. The mRNA for THANK was expressed at high levels by peripheral blood leukocytes, lymph node, spleen, and thymus and at low levels by small intestine, pancreas, placenta, and lungs. THANK was also prominently expressed in hematopoietic cell lines. The recombinant purified protein expressed in the baculovirus system had an approximate molecular size 20 kDa with amino-terminal sequence of AVQGP. Treatment of human myeloid U937 cells with purified THANK activated nuclear transcription factor-kappaB (NF-kappaB) consisting of p50 and p65. Activation was time- and dose-dependent, beginning with as little as a 1 pM amount of the cytokines and as early as 15 min. Under the same conditions, THANK also activated c-jun NH2-terminal kinase (JNK) in U937 cells. THANK also strongly suppressed the growth of tumor cell lines and activated caspase-3. Although THANK had all the activities and potency of TNF, it did not bind to the TNF receptors. Thus our results indicate that THANK is a novel cytokine that belongs to the TNF family and activates apoptosis, NF-kappaB, and JNK through a distinct receptor.