Despite aggressive conventional therapy for acute myelogenous leukemia (AML), it remains a very difficult disease to treat with long-term disease free survival rates of less than 40% in adults and less than 60% in children. With advances in molecular biology, AML has emerged as a highly heterogeneous group of disorders characterized by specific genetic alterations. These include chromosomal translocations and mutations in receptor tyrosine kinases encoding mutant proteins that are necessary in the pathogenesis of disease. With this insight into AML, there has been much interest in the development of targeted therapy against these mutant proteins. FLT3 is the most commonly mutated protein in AML and FLT3 mutations confer a worse prognosis. Several small molecule inhibitors of FLT3 have been designed and are currently being evaluated in clinical trials. The first specific aim of this proposal is to characterize all activating mutations in FLT3 and to determine the sensitivity of each mutation to the available FLT3 inhibitors. These data will have important implications in clinical trial design and patient inclusion criteria. The second specific aim of this proposal is to screen for mutations that confer resistance to FLT3 inhibitors and to characterize patterns of resistance. As patients are treated with FLT3 inhibitors, resistance will inevitably develop and it is crucial to anticipate resistance and have strategies in place to overcome it. The third specific aim of this proposal is to screen for novel mutations in patients with AML. Using high throughput sequencing of the activation loops and juxtamembrane domains of all tyrosine kinases, novel mutations may be identified which can then be further characterized as potential targets for therapy. Given the high morbidity and poor outcome associated with conventional therapy for AML, there is an urgent need to develop novel targeted therapies. This proposal aims to characterize FLT3 mutations in AML, to identify mutations that confer resistance to the available FLT3 inhibitors, and to identify novel mutations in AML as potential targets for therapy.