We used infection with an avirulent strain of Semliki Forest virus (aSFV) to induce NK cells in vivo, because it a) induces a potent NK cell response but no antiviral CD8 T cell response in C57Bl/6 mice [16], [19], and b) does not result in pathology in gzmAxB−/− or perf−/− mice (data not shown). This evidence concerns the gene CD8A and infection.