PRF1 and neoplasm: By using MHC class I-, MHC class II-, perforin-, B-cell- and IFN-γ- knock-out mice and CD4+ T cell-, CD8+ T cell- and NK cell- depleted mice, we demonstrated that CD4+ T cells and NK cells are the main anti-tumor effectors, and that IFN-γ is a major effector molecule.