PRF1 and neoplasm: During an immune response, cytotoxic T cells and NK cells can induce apoptosis of target cells through death receptor-mediated cytotoxicity, involving the death receptor ligands, tumor necrosis factor-related apoptosis-inducing ligand and Fas ligand, or by granule-mediated cytotoxicity, involving perforin and granzymes.2 During granule-mediated cytotoxicity, cell lysis is induced after tumor antigen recognition by secretion of cytotoxic pore-forming perforin.