A strong cell-mediated immune response is essential for protective immunity against TB.15–18 Class-II restricted CD4+ T cells, together with the antigen specific release of interferon-gamma (IFNγ) and tumour necrosis factor-alpha (TNFα) are necessary for protection19–24 and loss of CD4+ T cells increases the likelihood of succumbing to TB.25 IL-2 is important for central memory T cell responses26 and Class I restricted CD8+ T cells are also necessary for optimal protection.27,28 However, none of these immunological parameters alone correlate well with protection. The gene discussed is CD8A; the disease is tuberculosis.