It is also possible that CD4+ T cells could themselves directly kill tumor cells, e.g., through FasL/Fas interactions, similar to what has been described for killing of MHC IIPOS B lymphoma cells (33), or a perforin/granzyme B-dependent mechanism as described for killing of the MHC IIPOS B16 cells (37). The gene discussed is PRF1; the disease is neoplasm.