Bcl-x_L is required for the development of Eμ-myc–induced B cell lymphoma. Kaplan-Meier analysis of tumor development in mice reconstituted (as shown in Figure 2A) with fetal liver cells from Eμ-myc (n = 41) or Eμ-myc/bcl-x^−/− (n = 25) embryos. Mice were killed when deemed sick by an animal technician blinded to their genotype, and the presence of fetal liver donor-derived Εμ-myc pre-B or B lymphoma was confirmed by histological analysis and immuno-phenotyping. Cohorts of control mice (lethally irradiated C57BL/6-Ly5.1 coreconstituted with bcl-x^−/− (n = 21) or wt (n = 27) fetal liver cells plus rag-2^−/− bone marrow cells) were also followed. As anticipated, none developed pre-B or B-cell lymphoma within 70 weeks.