Abstract Abstract 1379 Biallelic CEBPA mutations and FLT3 length mutations are frequently identified in human acute myeloid leukemia (AML) with normal cytogenetics. However, the molecular and cellular mechanisms of oncogene cooperation remain unclear due to a lack of disease models. We have generated an AML mouse model using knock-in mouse strains to study cooperation of FLT3 ITD with commonly observed biallelic C/EBPa mutations which in this model are expressed from the original promoter and thereby at the right dose and at the correct level of cellular differentiation. Interestingly, Cebpa mutants with FLT3 ITD progressed to AML with shorter latency than cohorts with Cebpa mutations only. Before onset of leukemia, biallelic Cebpa mutations caused depletion of granulocyte-macrophage progenitors (GMPs), and expansion of megakaryocyte-erythroid progenitors (MkP, pMegE, pCFU-E) and HSCs. However, FLT3 ITD attenuated MegE progenitor frequencies while MPPs and myeloid progenitors (pGMP and GMP) were more frequently observed. These cellular changes are accompanied by an upregulation of hematopoietic stem cell and STAT5 target genes. Both MPP and GMP cells contain the potential to induce AML similar to corresponding cells in human AML samples showing that this AML model resembles human disease. By gene expression analysis in premalignant populations we further show a role of FLT3 ITD in activating genes involved in survival/transformation and chemoresistance. This murine AML model is perfectly suited to test the effects of chemotherapy and tyrosine kinase inhibitors blocking FLT3 ITD function on specific cellular populations to investigate their specific role in relapse of leukemia after complete remission. Disclosures: No relevant conflicts of interest to declare.