TCF3/PBX1.3 exhibits an immunophenotype and in vivo behavior consistent with pre–B-cell ALL. (A) Cultured TCF3/PBX1.3 cells (analyzed by flow cytometry) demonstrating expression of B-lineage receptors CD19⁺ and B220⁺ as well as the immature B-cell marker CD127⁺ (IL-7Rα) with cytoplasmic μ-chain present but without surface IgM expression. (B) Wright-Giemsa stain of TCF3/PBX1.3 demonstrating typical lymphoblastic morphology. (C) Spleens of C57BL/6 mice 2 weeks after injection of 1 × 10⁵ TCF3/PBX1.3 cells IV (top) or saline (bottom). (D) Histological sections of tissues from mice in panel C stained with hematoxylin and eosin. (E) CD45.1 congenic mice received 1 × 10⁵ TCF3/PBX1.3 cells (CD45.2) IV. At the time points indicated, bone marrow was collected analyzed by flow cytometry for CD45.2⁺B220⁺ TCF3/PBX1.3 cells. (F) TCF3/PBX1.3 cells were injected IV at decreasing doses into female C57BL/6 mice. Lethality was confirmed to be leukemia induced at autopsy. (G) Rag1^−/−, irradiated (250 cGy) C57BL/6, or unirradiated, immune-competent C57BL/6 mice received decreasing doses of leukemia (P < .004, Mantel-Cox test; irradiated vs healthy) or mice (P < .003, Mantel-Cox test; Rag1^−/− vs healthy).