To identify the molecular pathways through which WSX-1 represses Th1 cell terminal differentiation and thereby restricts the magnitude of the Th1 response during infection, we performed a phenotypic analysis of the splenic effector CD4+ T-bet+ T cells in WT and WSX-1−/− mice immediately prior to (day 9) and following (day 14) dysregulation of the Th1 response in WSX-1−/− mice. Here, CD4 is linked to infection.