The immune response against CMV infection seems to be affected by the site of infection, since various cell populations and effector mechanisms differentially contribute to virus control in different organs [14–16]: While IFNγ secreting NK cells have a more pronounced role than CD8 T cells in controlling the infection in the liver [14,15], an early, perforin-dependent NK cell mediated, and a late CD8 T cell based mechanism control virus load in the spleen [14]. The gene discussed is PRF1; the disease is infection.