In vivo depletion studies established that NK cells act to negatively regulate CD4+ and CD8+ T cell-dependent IFN-γ production and proliferation during murine cytomegalovirus infection, and they can mediate a similar effect on CD4+ T cell responses during lymphocytic choriomeningitis virus (LCMV) infection in β2-microglobulin deficient mice (Su et al., 2001). The gene discussed is CD4; the disease is cytomegalovirus infection.