We report here the isolation and characterization of a novel tumor necrosis factor-␣ (TNF-␣)-inducible gene, SCC-S2. Based on the nucleotide sequence, the SCC-S2 open reading frame contains a sequence in the amino terminus that shows a significant homology to death effector domain II of cell death regulatory protein, Fasassociated death domain-like interleukin-1-converting enzyme-inhibitory protein (FLIP). Unlike FLIP, the SCC-S2 open reading frame contains only one death effector domain and lacks the carboxyl-terminal caspaselike homology domain, raising the possibility that SCC-S2 may be a novel member of the FLIP family. SCC-S2 mRNA expression is found in most normal tissues and malignant cells. The steady state level of SCC-S2 mRNA is significantly induced by TNF-␣ in different tumor cells (TNF-␣ at 20 ng/ml for 3 h: A549, ϳ2-9-fold; SKOV-3, ϳ3-fold; PCI-04A, ϳ3-6-fold). TNF-␣ treatment (100 ng/ml, 4 h) of HeLa cells transiently transfected with FLAG epitope-tagged SCC-S2 cDNA or expression vector alone led to an increase in the number of apoptotic cells as compared with the untreated counterpart. Interestingly, however, SCC-S2 transfectants revealed a significant decrease in the number of apoptotic cells as compared with the vector transfectants (p < 0.001). These data implicate a role of SCC-S2 as a negative mediator of apoptosis in certain cell types.Increasing evidence suggests that apoptosis requires activation of members of the interleukin-1-converting enzyme-like family of cysteine proteases, also known as caspases. The caspase activation appears to be triggered by some members of the TNFR 1 superfamily, including TNFR1 (p55/CD120a) and TNFR2 (p75/CD120b), and Fas/Apo-1 (CD95). TNF binds to TNFR1, and FasL binds to Fas. TNFR1 and Fas, also known as death receptors, are characterized by the presence of a cytoplasmic sequence motif called the death domain, which interacts with the death domain of the adaptor molecules FADD and TNFR-associated death domain, recruiting them to the membrane. TNFR-associated death domain interacts with FADD, and FADD, in turn, associates with an apical caspase, FLICE (caspase 8/MACH/Mch5), through death effector domains (DEDs) present at the carboxyl terminus of FADD and the amino terminus of FLICE, resulting in the assembly of a receptor-associated death-inducing signaling complex. Death-inducing signaling complex-associated FLICE signals proteolytic activation of downstream caspases, ultimately leading to apoptosis (reviewed in Ref. 1). FADD mutant containing only the death domain or FLICE containing two DEDs can act as a dominant negative inhibitor of apoptosis (2-4). Because ligand activation of a death receptor does not lead to apoptosis in all cell types, it has been suggested that natural cell death inhibitory molecules may exist in certain cells. Indeed, FLICE-inhibitory proteins (FLIP, CASH, I-FLICE, and FLAME-1) containing two sequence motifs with significant homology to DEDs have been identified (5-9). FLIPs contain two DEDs in the amino terminus and are repres...