STAT1 and neoplasm: CD8+ CTLs kills tumor cells primarily through IFN-γ-mediated or perforin-mediated mechanisms.5 The IFN-γ, as a CD8+ CTL–derived cytotoxic cytokine, can initiate apoptosis in tumor cells by activating JAK-STAT1-caspase3 cascade.6 In addition, the perforin-mediated tumor killing mechanisms is achieved through degranulation of CD8+ CTLs, such as releasing degranulated cargos, which including the cytotoxic proteins perforin and granzymes.