Fzr1-intact and Fzr1-deficient B-ALL share similar pathological characteristics. (A) Experimental design for retrovirus-mediated introduction of Mycn into Fzr1-intact (Fzr1^f/f) or Fzr1-deficient (Fzr1^∆/∆) BM-MNCs and transplantation of the infected cells into WT recipient mice (see Materials and methods for experimental details). 5FU, 5-fluorouracil. (B) Flow cytometry–based determination of the proportion of GFP⁺ cells in the PB of recipient mice immediately and 9 weeks after transplantation of Mycn-overexpressing Fzr1-intact or Fzr1-deficient BM-MNCs. The data are presented as mean ± standard deviation (SD) (n = 3 mice per group). NS, not significant; P = .44. (C) Gross appearance of enlarged lymph nodes (left) as well as hematoxylin and eosin staining of affected LNs (middle) and BM (right) in mice given Fzr1-intact (top) or Fzr1-deficient (bottom) cells when they became moribund. Insets show higher-magnification views. Scale bars, 50 µm (low power) or 20 µm (high power). (D) Immunohistochemical staining of Ki67 in affected lymph nodes as in panel C. The percentages of Ki67⁺ cells are shown as mean ± SD determined for 3 mice of each group. Scale bars, 50 µm. (E) Representative PCR-based genotyping of sorted GFP⁺ cells from affected lymph nodes or BM of individual recipient mice. NC, negative control consisting of genomic DNA from BM cells of Mx1-Cre(−);Fzr1^f/f mice; PC, positive control, consisting of genomic DNA from BM cells of Mx1-Cre(+);Fzr1^f/f mice after pIpC injection.