Defining the developmental stages of B-ALL–initiating events. HSCs, CLP, proB, and preB cells from p19^ARF-null bone marrow were flow sorted and transduced with vectors coexpressing p210BCR/ABL and EGFP, followed by transplantation into sublethally irradiated wild-type recipients. (A) Peripheral blood of recipient mice was sampled at day 17 after transplantation for histological (top row) and flow cytometric analyses to assess the GFP expression and lineage distribution of leukocytes (center and bottom rows). (B) At day 23 after transplantation, bone marrow cells from the p19^ARF-null HSC primary recipients were flow sorted into 3 distinct cell groups, (1) B-lymphoid marker positive, (2) myeloid marker positive, and (3) lineage negative, followed by transplantation into sublethally irradiated secondary wild-type recipients. The plot shown was gated on EGFP⁺ and T-cell negative (CD3⁻) cell population. (C) Outcome of the secondary recipients that received different sorted cells from p19^ARF-null HSC primary recipients indicated in panel B.