Despite numerous studies and suggested markers, only four biomarkers were identified as having strong potential for diagnosing endometriosis: tumor necrotic factor α (TNF-a), matrix metalloproteinase-9 (MMP-9), tissue inhibitor of matrix metalloproteinase 1 (TIMP-1), and microRNA-451 (miR-451). Here, MMP9 is linked to endometriosis.