Acute promyelocytic leukemia (APL) cells invariably express aberrant fusion proteins involving the retinoic acid receptor ␣ (RAR␣). The most common fusion partner is promyelocytic leukemia protein (PML), which is fused to RAR␣ in the balanced reciprocal chromosomal translocation, t(15;17)(q22:q11). Expression of PML͞ RAR␣ from the cathepsin G promoter in transgenic mice causes a nonfatal myeloproliferative syndrome in all mice; about 15% go on to develop APL after a long latent period, suggesting that additional mutations are required for the development of APL. A candidate target gene for a second mutation is FLT3, because it is mutated in approximately 40% of human APL cases. Activating mutations in FLT3, including internal tandem duplication (ITD) in the juxtamembrane domain, transform hematopoietic cell lines to factor independent growth. FLT3-ITDs also induce a myeloproliferative disease in a murine bone marrow transplant model, but are not sufficient to cause AML. Here, we test the hypothesis that PML͞RAR␣ can cooperate with FLT3-ITD to induce an APL-like disease in the mouse. Retroviral transduction of FLT3-ITD into bone marrow cells obtained from PML͞RAR␣ transgenic mice results in a short latency APL-like disease with complete penetrance. This disease resembles the APL-like disease that occurs with long latency in the PML͞RAR␣ transgenics, suggesting that activating mutations in FLT3 can functionally substitute for the additional mutations that occur during mouse APL progression. The leukemia is transplantable to secondary recipients and is ATRA responsive. These observations document cooperation between PML͞RAR␣ and FLT3-ITD in development of the murine APL phenotype.A cute promyelocytic leukemia (APL) comprises 5-10% of cases of AML and is almost always associated with rearrangement of the retinoic acid receptor ␣ (RAR␣) gene as a consequence of balanced reciprocal chromosomal translocations. The t(15;17)(q22;q11.2) is the most common such translocation and most APL cells with this translocation express both the promyelocytic leukemia (PML)͞RAR␣ and RAR␣͞PML fusion proteins. Mouse models and cell culture assays have demonstrated that the PML͞RAR␣ fusion protein contributes to the leukemic phenotype in part by inhibiting differentiation and promoting survival of hematopoietic progenitor cells (1-6).Data from murine models and analysis of human APL genotypes indicate that the PML͞RAR␣ fusion protein is necessary, but not sufficient for the APL disease phenotype. Transgenic mice expressing PML͞RAR␣ under the control of the cathepsin G promoter can develop acute myeloid leukemia (AML) with many of the features of APL (1, 2, 7, 8). All transgenic animals expressing this transgene display a myeloproliferative syndrome characterized by elevated numbers of myeloid cells in the marrow and spleen. After a latency of 6-13 months, 15-20% of animals develop an APL-like disease. However, the penetrance increases to about 60% when both the PML͞RAR␣ and the reciprocal RAR␣͞PML cDNAs are expressed in early myeloid cel...