Effects of IL-3 treatment on lymphoid populations. IL-3–treated Jak3^−/− mice have thymic enlargement and increased numbers of thymocytes and peripheral T and B cells. (A) Thymi from 15-day-old mice are shown. (B) Thymocyte number from mice at 2 to 3 weeks of age. Control-treated Jak3^−/− mice (•; n = 17), IL-3–treated Jak3^−/− littermates (○; n = 16), and Jak3^+/+ mice (□; n = 13). (C) Flow cytometric analysis of splenocytes from 3-week-old mice. The percentage of positive cells is shown for each quadrant. This experiment is representative of 4 independent experiments.