Although early work utilized allogeneic CD4 cell clones due to their high frequency in normal mice, in 1985, CD4+ T cell clones were isolated after influenza A/Jap/57 infection that were able to lyse influenza-infected target cells expressing class II, but not class I MHC, suggesting that CD4 CTL activity developed after infection and subsequent in vitro culture (5). This evidence concerns the gene CD4 and infection.