Severe pro-B cell arrest in Fnip1^−/− mice. (A) Left bar graph: spleen weight (given as percentage of body weight) in Fnip1^−/− mice relative to littermate controls graphed as the mean ± SD; NS = no statistical significance, **P < .01 (Student t test), n = 9 (Fnip1^+/+), 17 (Fnip1^+/−), and 13 (Fnip1^−/−). Right graph shows the same for thymuses. n = 6 (Fnip1^+/+), and 8 (Fnip1^−/−). (B-D) Fluorocytometric analysis of CD4⁺ and CD8⁺ thymocytes (B), CD4⁺ and CD19⁺ splenocytes (C), and B1 (B220^highCD19^low) and B2 (B220^lowCD19^high) peritoneal cavity B cells (D). (E) Absolute number of pre–pro-B (B220⁺IgM⁻CD19⁻), pro-B (B220⁺IgM⁻CD19⁺CD43⁺CD25⁻), pre-B (B220⁺IgM⁻CD19⁺CD43⁻CD25⁺), and immature (B220⁺IgM⁺) B cells in BM of Fnip1^+/− and Fnip1^−/−mice. *P < .05, **P < .01 (Student t test), n = 4 for both strains. (F-H) Fluorocytometric analysis of B220 and IgM (F), B220 and CD43 (G), and BP-1 and HSA (H) expression in BM cells from Fnip1^+/−and Fnip1^−/−mice. Each analysis is representative of at least 3 independent experiments.