For example, after an infection in susceptible mouse strains, such as BALB/c and Swiss, an induction of a Th2-type immune response characterized by secretion of anti-inflammatory or regulatory cytokines, IL-10, IL4, IL- 5, IL-6, can inhibit the production of IL-12 cytokine – inductor of macrophage leishmanicidal action – and down-regulate the activity of the enzyme inducible nitric oxide synthase (iNOS) so, the nitric oxide (NO) and IFN-γ production are often impaired, allowing parasites to disseminate and persist in the host tissue causing severe damage6,22. The gene discussed is IFNG; the disease is infection.