The B-cell defect in c-Myb^Plt4/Plt4 mice is more severe in older mice. (A) Flow cytometry of adult (8-10 weeks old) c-Myb^+/+, c-Myb^Plt4/+, and c-Myb^Plt4/Plt4 mice. Top indicates total B cells; bottom, pro-B cells. Populations were identified as described in Figure 1. Plots are representative of 3 mice per genotype. (B) Flow cytometry of fetal liver cells from e16.5 embryos. Numbers in the plots represent the proportion of cells within the gated population expressed as the mean ± SD of 7 to 21 embryos per genotype. (Top) Numbers above the boxes indicate percentages of total B cells (B220⁺CD19⁺) among all live cells. (Bottom) Numbers above the boxes indicate percentages of pro-B cells (CD19⁺c-kit⁺) among all live cells. *P < .05; **P < .01; ***P < .001 in test group vs c-Myb^+/+ cells.