Murine mammary tumor virus-tet-controlled transcriptional activator (MMTV-tTA)/AML1-ETO double-positive mice were killed and the bone marrow was harvested. Colony assays were performed and replated serially through at least 13 generations in the absence of tetracycline. After 9 generations (Fig. 8A, black line), colonies were replated in the presence or absence of tetracycline. Cytocentrifugation was performed on the colonies generated and the slides were stained with Wright-Giemsa (Fig. 8C). To obtain differential counts for each sample, 100 cells were counted in triplicate (data shown here). This was repeated after 11 generations (Fig. 8A, purple line) with similar results (data not shown).