Increased survival of Rac1^TRac2^−/− icTCRβ⁻ DN4 thymocytes. Analysis of DN3, icTCRβ⁺ DN4, and icTCRβ⁻ DN4 thymocytes from mice of the indicated genotypes, showing (A) percentage of TUNEL⁺ cells as a measure of apoptosis, (B) ratio of Bcl2/Bax expression, and (C) surface levels of IL7Rα as judged by mean fluorescence intensity (MFI) of antibody staining. Statistically significant differences between Rac1^TRac2^−/− and WT mice are indicated (*P < .005; **P < .001).