Tumor necrosis factor (TNF) is a mediator of inflammation during immune responses and is produced by activated macrophages and T cells. It is synthesized as a membrane-bound homotrimer that is released from activated cells. Its functions are to inhibit viral replication, cause capillary leakage, induce cell apoptosis, and to recruit other immune cells to the infected area. Many humans infected with Sin Nombre (SN) virus develop an inflammatory pulmonary immunopathology caused by abundant production of TNF. In contrast, infected deer mice, the natural reservoir of SN virus, exhibit no pathology or inflammation in the lungs. Because of this, it is unlikely that deer mice produce appreciable amounts of TNF in response to SN virus infection. To begin to address this issue, we have cloned and sequenced the Tnf gene of deer mice. We obtained all but the final 9 nucleotides of Tnf and have determined that it is highly conserved with orthologous sequences.