CTSB and cancer: Since the incorporation of photosensitizers into nanomedicine makes them suitable for combinational therapies, the anti-tumor drug PTX was covalently linked to the polymer backbone via a cathepsin B-stimuli-responsive tetrapeptide Gly-Phe-Leu-Gly (GFLG), because cathepsin B, which is a lysosomal protease that is overexpressed in many cancer cells, is able to expedite the release of the covalently linked PTX [59].