Profound tumor-directed cytotoxicity is exhibited by tumor-infiltrating lymphocytes (TILs), accomplished through a number of mechanisms, including production of large amounts of proinflammatory cytokines such as TNF-α and IFN-γ; secretion of death-inducing granules containing granzymes, perforin, cathepsin C, and granulysin; and apoptosis via Fas-Fas ligand (FasL) activation of caspases and endonucleases. The gene discussed is PRF1; the disease is neoplasm.