CD4+ T cells are known to be important in the protective immune response to blood-stage malaria (Suss et al., 1988; Kumar et al., 1989; Podoba and Stevenson, 1991; Good and Doolan, 1999), and we confirmed that CD4+-T-cell-depleted mice displayed greater parasitemia and a higher mortality rate (Figure 1A). The gene discussed is CD4; the disease is malaria.