(A and B) B6 and Irf3/7 / mice were injected intraperitoneally with 200 nmol c-di-GMP, and (A) serum IFN was measured by bioassay 18h later, or (B) 36 h after injection, splenic NK responsiveness to YAC-1 target cells (or PBS control) was measured ex vivo by intracellular staining for IFN-. (C) B6 and Irf3/7 / mice were injected intraperitoneally with HSA ± 200 nmol c-di-GMP, and 2 wk later serum IgG1 specific for HSA was determined by ELISA. The experiments in A and B were repeated twice, and the experiment in C was performed once. Horizontal bars represent means. **, P < 0.01; and *, P < 0.05 (Student's t test) . respond well to c-di-GMP. Given the marked chemical dissimilarities among DNA, RNA, and c-di-GMP, it is perhaps not surprising that these nucleic acids appear to be recognized by different sensors. Indeed, we favor the idea that there are multiple cytosolic sensors for nucleic acids leading to induction of type I IFNs. The specificities of these various sensors will require further dissection. For example, although DAI knockout cells still responded to c-di-GMP, it remains formally possible that DAI is just one of several redundant sensors for c-di-GMP. These possibilities can be addressed once additional cytosolic nucleic acid sensors are identified.