Phenotypes of p85α^−/− and Btk^−/− mice. (A) Splenocytes from the indicated mice were stained with anti-B220 and anti-AA4.1, and analyzed on a FACSAria. The percentages of immature B cells (B220⁺AA4.1⁺) and mature B cells (B220⁺AA4.1⁻) are indicated (left). Results are presented as mean plus or minus SD (percentages of heterozygotes) (right). Significant differences between p85α^+/− and p85α^−/− mice or Btk^+/− and Btk^−/− mice are indicated. *P < .01; **P < .05. (B) Analysis of B220⁺AA4.1⁺ immature B cells for surface expression of IgM and CD23: T1 represents IgM⁺CD23^lo; T2, IgM⁺CD23⁺; and T3, IgM^loCD23⁺. Among B220⁺AA4.1⁺ cells, the percentages of the T1, T2, and T3 subsets are indicated. (C) Splenocytes from the indicated mice were stained with anti-CD23, anti-CD21, and anti-IgM. Among CD23⁺ (top panels) and CD23⁻ (bottom panels) lymphocyte populations, the percentages of CD23⁺CD21^intIgM^lo FO B cells identified as the cells in the square region (top panels) or CD23⁻CD21^hiIgM^hi MZ B cells identified as the cells in the square region (bottom panels) are indicated. Data are representative of 6 mice each for p85α^+/− and p85α^−/− mice on a BALB/c background or 4 mice each for Btk^+/− and Btk^−/− mice on a (C57BL/6 × 129/Sv) mixed background.