MMP9 and prostate cancer: Numerous studies providing the important roles of MMPs in CRPC metastasis mechanisms have been reported: MMP-2 and MMP-9 protein have been associated with aggressive CRPC and acted as significant prognostic factors in human prostate cancer [20–22]; downregulation of MMP2-/-9 protein levels could reduce the migration and invasion of the treated CRPC cells [23–25].