B-ALL blasts exhibit phenotypic infidelity. (A) FACS profiles of leukemic cells in the spleen of recipients of BCR-ABL/GFP–transduced pro-B cells. Leukemic GFP⁺ cells included pro-B cells, pre-B cells, sIgM⁺ B cells, and more immature pre/pro-B cells (CD45R⁺CD43⁺CD24⁻IgM⁻). (B) CLPs were isolated from the bone marrow of wild-type mice (n = 4), transduced with BCR-ABL/GFP, and transplanted into Rag1^−/− recipients. Four weeks after the primary transplant, total splenocytes and leukemic GFP⁺ cells with a CLP, pre/pro-B-, pro-B-, and pre-B-cell phenotype were isolated from diseased primary recipients (n = 2) and each population was transplanted into 4 secondary Rag1^−/− recipients, respectively. All secondary recipients developed B-ALL by day 19 after transplantation. Note that leukemic cells in recipients of GFP⁺ pro-B and pre-B cells had phenotypes characteristic of more immature stages of B-cell development.