Both CD4+ and CD8+ T cells from Itk-/- mice upregulate the key transcription factor Eomes, which we found is critical for both GVHD and GVL, since Itk-/- Eomesflox/flox CD4cre+ T cell donors (deficient in both Eomes and ITK) did not mount a cytotoxic response against primary leukemia cells or clear tumor cells, both in vitro and in vivo. The gene discussed is ITK; the disease is neoplasm.