Suppressor of Cytokine Signaling-1 (SOCS-1) is an essential physiological inhibitor of IFN-γ signaling. Mice lacking this gene die in the early postnatal period from a disease characterized by hyperresponsiveness to endogenous IFN-γ. The SOCS box is a C-terminal domain shared with over 30 other proteins that links SOCS proteins to an E3 ubiquitin ligase activity and the proteasome, but whether it contributes to inhibition of cytokine signaling is currently disputed. We have deleted only the SOCS box of the SOCS-1 gene in mice and show that such mice have an increased responsiveness to IFN-γ and slowly develop a fatal inflammatory disease. These results demonstrate that deletion of the SOCS box leads to a partial loss of function of SOCS-1.