Rag1−/− pro-B cells were purified using anti-CD19 magnetic beads (Miltenyi). Cells were cultured at 1×106 cells/ml in 96-well plates in the presence of 10 ng/ml IL-7 (Peprotech). After 24, 48, and 72 hours, proliferation was measured by flow cytometry using AccuCount 5.27 μm Blank Particles (Spherotech). Q-VD-OPH (Sigma Aldrich) was used at 100 μM. For transduction, Rag1−/− pro-B cells were cultured for 24 hours prior to transduction with MLP or MLP-shp53 retroviral vectors, as previously described (19 (link), 25 (link)).