Intrinsic requirement of Stat5a/b for generation of Foxp3⁺ CD4 T cells. Irradiated C57BL/6 Rag2^−/− CD45.1⁺ congenic mice were reconstituted with 2 × 10⁶ total fetal liver cells from CD45.2⁺ WT (left panels) or Stat5^−/− (right panels) mice. After reconstitution (8 weeks), cell populations in the thymus (A) and spleen (B) were analyzed. Donor-derived CD4 SP T cells were analyzed for CD25 and Foxp3 expression. Numbers indicate the percentage of CD25⁺ or FoxP3⁺ cells delineated by the rectangles.