Stat5a/b are critical for the generation of thymic Foxp3⁺ CD4⁺ T cells and maintenance of peripheral Foxp3⁺ CD4 T⁺ cells. CD25 and Foxp3 expression were analyzed by flow cytometry in CD4 SP thymocytes (A-C) and splenocytes (D-F) from 4- to 6-week-old Stat5^+/+ and Stat5^ΔN mice (A,D), Stat5^−/− mice (B,E), and Jak3^−/− and Il2rg^−/− mice (C,F). Numbers indicate the percentage of CD25⁺ or FoxP3⁺ cells delineated by the rectangles.