The main Th1 effector cytokine IFN-γ plays an essential role in the induction of GvHD as grafts of IFN-γ-gene knockout donors could not cause GvHD in recipient mice (43) and, in turn, GvHD could not be induced in IFN-γ-signaling deficient mice (44). The gene discussed is IFNG; the disease is graft versus host disease.