Thus FlipFlop-origin CD4/MHC-II cytotoxic T cells do function in vivo but they do not protect against virus infection, partly because they cannot eliminate MHC-II– parenchymal cells that are infected with virus and partly because they actively inhibit propagation of protective antiviral immune responses that are dependent on MHC-II+ virus-presenting cells. This evidence concerns the gene CD4 and viral infectious disease.