CTSS and cancer: Epithelial to mesenchymal transition (EMT) followed by loss of adhesion proteins (e.g., cadherins), degradation of the extracellular matrix (ECM) by matrix metalloproteinases (MMPs), cathepsins and the urokinase-type plasminogen activator (uPA) system, as well as co-option of factors from the tumor microenvironment to support metastasis (e.g., cytokines TGF-β and SDF1), are all processes that provide cancer cells with the ability to invade blood vessels and migrate to distant sites in the body.