The generation of mice lacking the expression of the IRF3 transcription factor ( Irf3 −/− mice) has revealed its crucial role in the activation of the type I IFN response. The Bcl2l12 gene, encoding Bcl2L12 protein structurally related to the Bcl-2 family, was found to almost overlap with the Irf3 gene, and the null mutation previously introduced into the Irf3 allele resulted in the functional inactivation of the Bcl2l12 gene; therefore, the mice are correctly termed Irf3 −/− Bcl2l12 −/− mice. Embryonic fibroblasts from Irf3 −/− Bcl2l12 −/− mice ( Irf3 −/− Bcl2l12 −/− MEFs) showed resistance to DNA damage-induced apoptosis, accompanied by impaired caspase cleavage. This apoptotic defect in Irf3 −/− Bcl2l12 −/− MEFs was rescued by the ectopic expression of Bcl2L12, but not IRF3. The Bcl2L12-mediated apoptotic response depended on the cell type and extracellular stimulus. In contrast, the previously reported defect in the induction of type I IFN genes by nucleic acids in Irf3 −/− Bcl2l12 −/− MEFs was rescued by expressing IRF3, but not Bcl2L12. Thus, our present study revealed, on the one hand, a cell type-dependent proapoptotic function of Bcl2L12 and, on the other hand, confirmed the essential role of IRF3 in type I IFN response.