In the present study, we evaluated the roles of NO and the NOS system in pulmonary fibrosis by using mice lacking all three NO synthases, n/i/eNOS−/− mice, and showed that the lack of all NOS led to a deterioration of the fibrotic changes in the lungs of mice with BLM-induced pulmonary fibrosis. The gene discussed is NOS1; the disease is pulmonary fibrosis.