Mossman and Coffman defined two functionally distinct CD4+ T-cell subsets, Th1 and Th2, based on their functions and cytokine secretions (178, 179), and others explained differential function of Th1 and Th2 CD4+ subsets in the outcome of murine L. major infection: mice that effectively overcome the infection primarily demonstrate a Th1 type of immune response with high levels of IFN-γ production but little IL4, and mice with fatal, disseminating disease have a Th2 type of immune response, including a significant amount of IL4 and IL5 and lower levels of IFN-γ (177, 180–182). This evidence concerns the gene IFNG and infection.