Defective expression of CD25 in developingHoxa-9^−/− T cells. (A) Schematic representation of normal differentiation stages within the TN population of T cells, correlated with FACS scan profiles. The curved arrow points to the stage at which TCRβ rearrangement is largely completed. (B) FACS analyses of day-15.5 fetal thymocytes was performed using antibodies against CD25 and CD44. A fourfold to fivefold reduction in the percentages of CD25⁺ cells (both CD44⁺ and CD44⁻) was seen in the mutant thymuses. (C) Absolute number of thymocyte subsets in fetal thymuses. Based on FACS analysis of 3 separate litters (representing 9 mutant and 12 control animals), there is an absolute decrease in all four major subsets of T cells inHoxa-9^−/− thymuses, with a 25.9-fold decrease in the CD25⁺ compartments. (D) A representative FACS analysis of adult CD4⁻CD8⁻ thymocytes. The percentage of CD25⁺ cells is decreased in the mutant thymus (right panel). Similar results were seen with 2 additionalHoxa-9^−/− thymuses.