B-cell reconstitution in FL^−/−mice after transplantation of fetal liver cells or BM cells from 2 week old mice. Lethally irradiated adult WT and FL^−/− mice were transplanted with 2 × 10⁵ unfractionated day 14.5 fetal liver cells (A-C) or 5 × 10⁵ unfractionated 2-week-old BM cells (D,E), from WT and FL^−/− mice, respectively. Donor- and recipient-derived blood cells could be separated based on expression of different CD45 isoforms (Materials and methods). (A,B) Donor-derived PB B-cell and T-cell reconstitution 3 and 16 weeks after fetal liver cell transplantation. Mean (SEM) values from 9 recipient mice of each genotype. (C) At 17 weeks post-transplantation, recipient mice were killed, and BM cells were counted and analyzed for donor-derived B-cell progenitors. Mean (SEM) values for total number of each progenitor subset (per 2 tibiae and 2 femora) of 6 mice per genotype. (D,E) Donor-derived PB B-cell and T-cell reconstitution at 4 and 14 weeks after transplantation of BM from 2-week-old mice. Mean (SEM) values from 7 recipients of each genotype. *P < .05, **P < .01, ***P < .001, ns indicates not significant compared with WT mice.