At present, the main biomarkers of pulmonary fibrosis are as follows: MMP-9 [21], KL-6 [22], S100A6 [23], and MMP7 [21], but as we know, pulmonary fibrosis is an extremely complex disease, and a single biomarker is an inaccurate representation of pulmonary fibrosis status. The gene discussed is MMP9; the disease is pulmonary fibrosis.