Second, the proliferation rate of the isolated fibroblasts from BC004004−/− mice decreased and the production of collagen in fibroblasts from BC004004−/− mice after TGF-β1 stimulation decreased substantially, indicating that the impaired function of fibroblasts contributes at least partially to attenuated pulmonary fibrosis phenotype of BC004004−/− mice. The gene discussed is TGFB1; the disease is pulmonary fibrosis.