In order to test the effect of IL-9 on the proliferation of fibroblasts, we isolated lung fibroblasts from wild-type mice and stimulated them with IL-9 alone or in the presence of TGF-β (TGF-β was used to mimic the lung microenvironment in pulmonary fibrosis) for 24 or 48 h, and the cell proliferation rate was determined by CCK8 assay. The gene discussed is IL9; the disease is pulmonary fibrosis.