Loss of Bcl-x_L causes a reduction in Εμ-myc transgene expressing pro-B, pre-B, and sIg+ B cells. Lethally irradiated C57BL/6-Ly5.1 mice were reconstituted (as shown in Figure 2A) with fetal liver cells from wt, bcl-x^−/−, Eμ-myc or Eμ-myc/bcl-x^−/− embryos. Bone marrow (A), peripheral blood (B), spleen (C) and lymph nodes (axillary, brachial, inguinal plus mesenteric; D) were harvested from pre-leukemic mice 8-12 weeks after reconstitution. Flow cytometric analysis was used to determine the absolute numbers of fetal liver donor–derived (Ly5.2⁺) total B lymphoid cells, pro-B cells (Ly5.2⁺B220⁺c-Kit⁺sIg⁻), pre-B cells (Ly5.2⁺B220⁺c-Kit⁻sIg⁻) and sIg⁺ B cells (Ly5.2⁺B220⁺c-Kit⁻sIg⁺) within these tissues. Data represent mean ± SEM from 4-6 recipient mice transplanted with fetal liver cells of each genotype. *P < .05 and **P < .01 denote statistically significant differences between Eμ-myc/bcl-x^−/− and Eμ-myc reconstituted mice.