MMPs play an important role in the process of tumor metastasis by degrading extracellular matrix to destroy the histological barrier around tumors and promote the invasion of cancer cells into adjacent tissues.[24] The results of this study showed that the levels of MMP-7 and MMP-9 in malignant pleural effusion were significantly higher than those in benign pleural effusion. The gene discussed is MMP9; the disease is cancer.