Concomitant loss of FLT3 and CSF1R expression leads to a severe reduction in fetal B-cell progenitors. (A) Representative FACS profiles of CSF1R and FLT3 coexpression on E13.5 FL ProB cells. Numbers represent mean percentage (± SEM) of CSF1R⁺FLT3⁺, CSF1R⁺FLT3^–, and CSF1R^–FLT3⁺ ProB cells (n = 16 embryos, 3 experiments). (B) Total FL cells in E14.5 FL of Wt control (light gray), Flk2^−/− (gray), and Flk2^−/−Csf1r^−/− (dark gray) embryos (n = 16-21 embryos per genotype). (C) (Left) Representative FACS profiles of ProB cells in E14.5 FL of Wt, Flk2^−/−, and Flk2^−/−Csf1r^−/− embryos. Numbers represent mean percentage of gated populations relative to total FL cells (4-5 experiments). (Right) Mean percentage (± SEM) of ProB in E14.5 FL of Wt control (light gray), Csf1r^−/− (black), Flk2^−/− (gray), and Flk2^−/−Csf1r^−/− (dark gray) embryos (n = 14-21 embryos per genotype). Statistical significance was tested between Csf1r^−/− and Csf1r^−/−Flk2^−/−, and between Flk2^−/− and Csf1r^−/−Flk2^−/−. Because of multiple comparisons, Dunn’s correction of P values was performed (see Methods). ***P < .001; *P < .05.