On the base of compelling evidence that both CD4+ (including multifunctional Th1 cells and central memory CD4+ T-cells) and CD8+ T-cells are key players in the immune response to leishmaniasis, researchers have focused considerable efforts on the development of prophylactic vaccines that elicit T-cell responses [14], [16], [23] with the premise that such interventions will confer protective effects. The gene discussed is CD4; the disease is leishmaniasis.