Foxp1-deficient T cells acquire an activated phenotype by a cell-autonomous mechanism. At 6 to 10 weeks after mixed bone marrow reconstitution and gating on congenic markers, the donor thymocytes and peripheral T cells in mixed chimeras were analyzed for (A) CD4 and CD8 profile; (B) expression of CD44 and CD62L; and (C) percentages of Foxp1^+/+Cd4^Cre and Foxp1^f/fCd4^Cre T cells recovered in the thymuses, spleens, and lymph nodes of the mixed chimeras. The thymic data are normalized to the ratios of Foxp1^+/+Cd4^Cre and Foxp1^f/fCd4^Cre B220⁺ cells in the bone marrow; the peripheral T-cell data are normalized to thymic T cells. Bars represent average ± SD. n = 10. **P < .01. (D) IL-2 and IFN-γ production in splenic CD4⁺ T and CD8⁺ T cells by intracellular staining after PMA plus ionomycin stimulation for 4 hours. (E) Cell size. All data are representative of 2 independent experiments.