C3H/HeJ inbred mice are defective in that they are highly resistant to endotoxic shock as compared with normal responder mice. Their B cells and macrophages do not respond significantly when exposed to lipopolysaccharide (LPS), whereas cells from the responder mice do. Using a functional assay, we previously isolated a cDNA, which encodes for Ran/TC4 GTPase. We now show that this gene is mutated in C3H/HeJ mice, which accounts for their resistance to endotoxin stimulation. Sequence analysis of independent mutant Lps^d/Ran cDNAs isolated from splenic B cells of C3H/HeJ mice reveals a consistent single base substitution at position 870, where a thymidine is replaced with a cytidine. In situ hybridization maps the Lps^d/Ran cDNA to mouse chromosome 4. By retroviral gene transfer, the wild-type Lpsⁿ/Ran cDNA but not the mutant Lps^d/Ran cDNA can restore LPS responsiveness of C3H/HeJ cells. Adenoviral gene transfer in vivo with the mutant Lps^d/Ran cDNA but not the wild-type Lpsⁿ/Ran cDNA rescues endotoxin-sensitive mice from septic shock. Thus Lps/Ran is an important target for LPS-mediated signal transduction, and the Lps^d/Ran gene may be useful as a therapeutic sequence in gene therapy for endotoxemia and septic shock.