CTSS and cancer: Cysteine proteases, such as cathepsins and papain, are proteolytic enzymes that are expressed widely in tissues and have numerous functions, including inflammatory tissue destruction, modulation of the immune response, tissue remodeling, and induction of monocyte and cancer cell migration.1, 2 Cathepsins are intracellular cysteine proteases that function in protein degradation in lysosomes3 and secretory granules.