Metalloproteinase (MMP)-2 and MMP-9 are expressed in 69% (overexpression in 49%) and 49% (overexpression in 36%) of patients with CUP, respectively.84 These expression levels are decreased by the knockdown of proteasome subunit β type-4 (PSMB4) through deactivating the proteasome cascades.86 The expression of MMP-2 and MMP-9 is not associated with tumour differentiation, clinical subgroups, response to treatment or survival.84 Metastasis-suppressor genes are genes that decrease the capability of the malignant clone to degrade the surrounding extracellular matrix. This evidence concerns the gene MMP9 and neoplasm.