Low numbers of B cells in the patient and early B cell development arrest in Pax5 ^R31Q/E242 * and Pax5 ^R31Q/− mice. (A) Analysis of peripheral blood from the patient, the mother, and a control. T (CD3⁺), B (CD19⁺), natural effector B (CD19⁺CD27⁺IgD⁺), transitional B (CD19⁺CD27⁻IgD⁺CD38⁺CD24^dim), and naive mature B (CD19⁺CD27⁻IgD⁺CD38⁻CD24⁻) cells were identified by flow cytometry. Numbers refer to the percentage of cells in the indicated gate. One of three experiments is shown. (B) Flow-cytometric analysis of T (CD3⁺) and B (CD19⁺) cells in the blood from 8–10-wk-old mice of the indicated genotypes (upper panel). The frequencies of T and B cells are shown as mean values with SEM (lower panel; n ≥ 5 per genotype). (C) Flow-cytometric analysis of B cell development in the bone marrow of 3–4-wk-old mice of the indicated genotypes. Absolute numbers of total B, pro-B, pre-B, and immature (imm) B cells are shown as mean values with SEM (n ≥ 8 per genotype). Definitions of the different cell types are in Fig. S1 C and Materials and methods. (D) Analysis of Pax5 expression in pro-B cells of the indicated genotypes. Pax5 levels were determined by intracellular staining combined with flow-cytometric analysis. ANOVA with Tukey’s or Dunnett’s T3 multiple comparisons test (B and C); *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001. For detailed statistical information, see Table S6. Each dot (B and C) corresponds to one mouse.