Since the expression of full length AML1-ETO does not lead to the development of leukemia [19]–[21], we took advantage of an alternatively spliced isoform of the AML1-ETO transcript, AML1-ETO9a, which induces an acute myeloid leukemia in mice with a high penetrance [21]. The gene discussed is RUNX1; the disease is acute myeloid leukemia.