The activation of innate immune responses is critical to host defense against microbial infections, wherein nucleic acid-sensing pattern recognition receptors recognize DNA or RNA from viruses or bacteria and activate downstream signaling pathways. In a search for new DNA-sensing molecules that regulate innate immune responses, we identified RNA-binding motif protein 3 (RBM3), whose role has been implicated in the regulation of cell growth. In this study, we generated Rbm3-deficient (Rbm3(-/-)) mice to study the role of RBM3 in immune responses and cell growth. Despite evidence for its interaction with immunogenic DNA in a cell, no overt phenotypic abnormalities were found in cells from Rbm3(-/-) mice for the DNA-mediated induction of cytokine genes. Interestingly, however, Rbm3(-/-) mouse embryonic fibroblasts (MEFs) showed poorer proliferation rates as compared to control MEFs. Further cell cycle analysis revealed that Rbm3(-/-) MEFs have markedly increased number of G2-phase cells, suggesting a hitherto unknown role of RBM3 in the G2-phase control. Thus, these mutant mice and cells may provide new tools with which to study the mechanisms underlying the regulation of cell cycle and oncogenesis. (C) 2011 Published by Elsevier Inc.