Pre-B cells from Arf^−/− but not Ink4a^−/− mice can initiate B-ALL. (A) Pro-B and pre-B cells from wild-type (4), Arf^−/− (n = 2), and Ink4a^−/− (n = 2) mice were purified, transduced with retroviral vectors containing GFP only or BCR-ABL/GFP and 2.0 × 10⁴ cells were seeded in a single well in culture. The number of cells harvested 7 days later is indicated. (B) Pre-B cells were isolated from the bone marrow of wild-type (n = 2), Ink4a^−/− (n = 2), or Arf^−/− (n = 2) mice, transduced with BCR-AB/GFPL, and 80 000 cells were transplanted into 3 Rag1^−/− recipients per group. All recipients of the Arf^−/− pre-B cells developed B-lineage leukemia by day 22 posttransplantation, whereas wild-type or Ink4a^−/− pre-B cells showed no signs of disease.