The v-Rel oncoprotein must be continuously expressed to prevent the apoptosis of transformed lymphoid cells, and also inhibits TNFα-induced cell death. A tetracycline-regulated cell system was used to characterize the functions necessary for the anti-apoptotic activity of Rel proteins. v-Rel mutants defective for DNA binding or transactivation showed no protective effect. Similarly, whereas the transcription-competent c-Rel and RelA proteins inhibited TNFα-induced cytolysis, the transactivation-negative p50/NF-κB1 did not. Importantly, this study is the first to show that c-Rel can also confer significant protection from Fas-mediated cell death. Since the TNFR1-and Fas-signaling pathways involve some intermediates that are common and others that are unique to each pathway, these findings indicate that c-Rel may regulate the expression of genes that function to antagonize either or both death-signaling pathways.