PLG and cancer: MMP-9 is secreted by various cells in an inactivated form as a 10 kDa propeptide, which could be activated by other MMPs or tissue plasminogen activator (tPA)-plasmin system, due to its biological function of cleaving gelatin, which promotes malignant cell motility [3, 4], MMP-9 was regarded as an important prognostic marker and therapeutic target of cancer in many studies.