Impaired B-cell recovery after chemotherapy-induced myeloablation of FL^−/− mice. The 9- to 15-week-old WT and FL^−/− mice were treated with a single dose of 5-FU (150 mg/kg mouse weight). PB cellularity and lineage composition were determined at different time points post-treatment. At 7 days before treatment, PB was analyzed to establish the baseline levels of all blood cell lineages (depicted as day 0 and indicated by a dotted line). Data are expressed as mean (SEM) number of Mac-1⁺ myeloid cells (A), CD4⁺ T cells (B), CD8⁺ T cells (C), and B220⁺IgM⁺ B cells (D) per mL blood. (E) At day 68-83 of treatment, BM cells were harvested and counted from 5-FU–treated mice and B-cell progenitor subsets analyzed (“Materials and methods”). Data are expressed as mean (SEM) number of each progenitor subset (from 2 tibiae and 2 femora) from 11-14 treated mice per genotype from 2 experiments. ***P < .001 compared with WT mice.