In contrast, CD4+YFP+GFP− T cell–derived memory cells responded rapidly during secondary infection, expanding to a significantly greater extent than CD4+YFP+GFP+ T cell–derived memory cells in the spleen (fold change expansion of 2.3 versus 0.8) and liver (fold change expansion of 5.8 versus 1.0) by day 4 of infection (Fig. 4A). This evidence concerns the gene CD4 and infection.