CTSS and neoplasm: Cathepsin is secreted on the cell surface in the highly acidic tumour microenvironment and is similar to matrix metalloproteinases in its role in the degradation of multiple extracellular matrix (ECM) proteins and basement membranes, thereby promoting tumour cell invasion and metastasis [42], and a number of cathepsins have been found to activate MMP-9 through the proteolysis of its pro-domain [43].