Immature B-cell progenitors can generate B-ALL. (A) All recipients of BCR-ABL/GFP–transduced HSCs (n = 8; 11 000 cells/mouse), CLPs (n = 8; 25 000 cells/mouse), and pro-B cells (n = 8; 65 000 cells/mouse) developed leukemia by day 42 posttransplantation. Recipients of BCR-ABL/GFP–transduced pre-B cells (n = 12; 100 000 cells/mouse) showed no sign of disease. Results pooled from 3 independent experiments in which 2 to 4 recipients were reconstituted with the indicated cell population in each experiment. (B-C) FACS profiles of leukemic cells in the spleen and bone marrow of recipients of BCR-ABL/GFP–transduced CLPs (B) and pro-B cells (C). GFP⁺ cells purified from mice by FACS were large blasts (bar represents 20 μm).