Tetrapeptide linkers such as Gly-Phe-Leu-Gly (GFLG) or Gly-Gly-Phe-Gly (GGFG), a tetrapeptide linker composed of four amino acids, are cleaved by proteases, including cathepsins, within the tumor microenvironment or intracellular organelles. Here, CTSS is linked to neoplasm.