The essential role of IRF-1 and the substantial role of iNOS in macrophage cytotoxicactivity lead us to investigate the role of these molecules in tumor growth invivo. Notably, we found that IRF-1 deficient mice, when inoculated with L929tumor cells, were not able to produce NO and were extremely susceptible to tumor cellsgrowth when compared with WT mice. The gene discussed is NOS2; the disease is neoplasm.