AbstractAs reported previously, AML1-ETO knock-in mice were generated to investigate the role of AML1-ETO in leukemogenesis and to mimic the progression of t(8;21) leukemia. These knock-in mice died in midgestation because of hemorrhaging in the central nervous system and a block of definitive hematopoiesis during embryogenesis. Therefore, they are not a good model system for the development of acute myeloid leukemia. Therefore, mice were generated in which the expression of AML1-ETO is under the control of a tetracycline-inducible system. Multiple lines of transgenic mice have been produced with the AML1-ETO complementary DNA controlled by a tetracycline-responsive element. In the absence of the antibiotic tetracycline, AML1-ETO is strongly expressed in the bone marrow of AML1-ETO and tet-controlled transcriptional activator double-positive transgenic mice. Furthermore, the addition of tetracycline reduces AML1-ETO expression in double-positive mice to nondetectable levels. Throughout the normal murine lifespan of 24 months, mice expressing AML1-ETO have not developed leukemia. In spite of this, abnormal maturation and proliferation of progenitor cells have been observed from these animals. These results demonstrate that AML1-ETO has a very restricted capacity to transform cells. Either the introduction of additional genetic changes or the expression of AML1-ETO at a particular stage of hematopoietic cell differentiation will be necessary to develop a model for studying the pathogenesis of t(8;21).