L. donovani-infected mice mount a vigorous anti-leishmanial type 1 CD4+ and CD8+ T cell response that leads to control of the infection, primarily through the upregulation of inducible nitric oxide synthase 2 (iNOS or NOS2) and generation of nitric oxide (NO) in the spleen and liver [2]–[4]. The gene discussed is NOS2; the disease is infection.