With the emerging role of NF-?B in cancer it is important that its responses to stimuli relevant to tumor progression and therapy are understood. Here, we demonstrate that NF-?B induced by cytotoxic stimuli, such as ultraviolet light (UV-C) and the chemotherapeutic drugs daunorubicin/doxorubicin, is functionally distinct to that seen with the inflammatory cytokine TNF and is an active repressor of antiapoptotic gene expression. Surprisingly, these effects are mediated by the RelA(p65) NF-?B subunit. Furthermore, UV-C and daunorubicin inhibit TNF-induced NF-?B transactivation, indicating that this is a dominant effect. Consistent with this, mechanistic studies reveal that UV-C and daunorubicin induce the association of RelA with histone deacetylases. RelA can therefore be both an activator and repressor of its target genes, dependent upon the manner in which it is induced. This has important implications for the role of NF-?B in tumorigenesis and the use of NF-?B inhibitors in cancer therapy.