Regarding other molecules participating in pleural effusion, matrix metalloproteinases (MMP) and their antagonists, the tissue inhibitors of metalloproteinases (TIMP) are present in pleural effusions, with some of these molecules, such as MMP-1, MMP-2 and TIMP-1 being constitutively present in the pleural fluid, while MMP-9 and TIMP-2 are found in specific diseases states [52]. This evidence concerns the gene MMP9 and Pleural effusion.