Following infection with L. major (LV39), IL-4−/− or IL-4Rα−/− mice on a BALB/c genetic background were able to control lesion size and the levels of IFNγ present in draining lymph node (dLN) cells was either very low or remained unchanged compared to that observed in BALB/c wild-type mice (6, 7). The gene discussed is IFNG; the disease is infection.