Histology of the acute leukemia disease in bcr-ABL/bcr⁺ and bcr-ABL/bcr⁻ chimeric mice. Histologic analysis was performed when the mice were moribund. The spleen from bcr-ABL/bcr⁺ and bcr-ABL/bcr⁻ chimeric mice that developed an acute leukemia shows the effacement of the normal spleen architecture (compared with control chimeric mice). Similar cells invade visceral organs such as the liver or the heart. Control chimeric mice were generated by injecting wild-type E14 cell clone or ES-E14 cells in which one bcr allele had been modified with the bcr targeting vector into blastocysts. Although the acute leukemia disease in bcr-ABL/bcr⁺ and bcr-ABL/bcr⁻ chimeric mice was very similar, the level of leukemic infiltration shown by histologic analysis was not identical (original magnification ×50).