Likewise, mice genetically deficient in CD4+ or CD8+ T cell function were extremely susceptible to infection [42], and it was shown that T lymphocytes act by producing type 1 cytokines, such as IFNγ, and CD8+ T cell cytotoxicity mediated by perforin is important for resistance against infection [9,25,43,44]. The gene discussed is IFNG; the disease is infection.