CTSB and neoplasm: To take advantage of overexpressed enzymes, many PTs have been designed with protease-sensitive linker oligopeptide moieties (e.g., GFLG (Gly-Phe-Leu-Gly) and GLFG (Gly-Leu-Phe-Gly)) designed to be stable in the blood but rapidly cleaved by lysosomal enzymes (cathepsin B or D and others) within the tumor stroma [139].