The NF-κB/IκB signaling pathway is a critical regulator of cell survival in cancer. Here, we report that combined down-regulation of growth arrest- and DNA-damage-inducible proteins (GADD)45α and γ expression by NF-κB is an essential step for various cancer types to escape programmed cell death. We demonstrate that inhibition of NF-κB in cancer cells results in GADD45α- and γ-dependent induction of apoptosis and inhibition of tumor growth. Inhibition of GADD45α and γ in cancer cells by small interfering RNA abrogates apoptosis induction by the inhibitor of NF-κB and blocks c-Jun N-terminal kinase activation, whereas overexpression of GADD45α and γ activates c-Jun N-terminal kinase and induces apoptosis. These results establish an unambiguous role for the GADD45 family as an essential mediator of cell survival in cancer cells with implications for cancer chemotherapy and novel drug discovery.