PRF1 and hepatocellular carcinoma: Although the killing mechanism of cytotoxic T-lymphocytes against HCC cells was not investigated, the generally accepted mechanism is their expression and regulated secretion of potent toxins, including pore-forming protein perforin and serine protease granzymes after formation of immunological synapse from the binding of T-cell receptor (TCR) to peptide ligand presented on MHC molecule on the cancer cells.