Reconstitution of IL-4 production by retroviral Vav1, but not c-Maf, expression. Naive CD4⁺ T cells from Vav1^–/– or wild-type mice were infected with a Vav1-expressing (A) or a c-Maf–expressing (B) and control (GFP) retrovirus 18 hours after primary activation by anti-CD3 plus anti-CD28 antibodies plus IL-2 under Th2-inducing conditions. Cells were harvested on day 4, restimulated with anti-CD3 plus anti-CD28 antibodies, and IL-4–producing cells were enumerated by ICCS 8 hours later. Data are displayed as dot plots showing GFP (horizontal axis) versus intracellular IL-4 (vertical axis) expression. The numbers show percentages of IL-4⁺ and IL-4^– T cells within the GFP⁺ population (= 100%). The data shown are representative of 3 independent experiments.