Increased caspase activation and apoptosis in Fnip1^−/− B cells. (A) Bar graph showing the frequency of dead cells within pro-B (B220^lowIgM⁻CD25⁻) and pre-B (B220^lowIgM⁻CD25⁺) populations from Fnip1^+/+ and Fnip1^−/− mice as measured by DAPI staining. Mean ± SD from 4 independent experiments, (pro-B) P = .1, (pre-B) P < .02, Student t test. (B) Flow cytometric measurement of caspase activity in Fnip1^+/+ and Fnip1^−/− pre-B cells from interleukin 7 ex vivo cultures. Twenty-four hours after interleukin 7 removal cells were stained with DAPI and FAM, a carboxyfluorescein moiety that becomes covalently linked to the active site of caspases. (C) Flow cytometric analysis of BM and (D) spleen cells from Fnip1^−/− mice reconstituted with an Eμ-Bcl2 transgene. BM cells were stained with anti-B220 and anti-IgM antibodies; staining for splenocytes was with anti-CD4 and anti-CD19. Each FACS analysis is representative of 3 independent experiments.