AML is a devastating blood tumor, the most common type of leukemia in adults, a disease that continues to have the lowest survival rate within leukemia. Nearly 45,000 people are diagnosed each year in the US, and the current 5-year survival frequency is only 24% with an almost 50% relapse rate after treatment. AML is a part of a wider family of myeloid neoplasms that include diverse but related diseases like myelodysplastic syndromes (MDS), myeloproliferative neoplasms (MPN) and chronic myelomonocytic leukemia (CMML), a disease that frequently develops into AML. Currently, there are no targeted therapies for most of these diseases, including AML, making the study of the molecular mechanisms of their induction and progression of great significance. We have recently identified novel somatically acquired mutations inactivating the Notch signaling pathway in patients with CMML. Interestingly, these Notch mutations do not act in isolation but they co-occur with additional genomic "hits", including mutations on the TET2, ASXL1, KRAS and JAK2 genes. Our published and preliminary data presented here demonstrate that: a) The Notch pathway is "switched-off" in myelo-monocytic leukemias, including AML, b) Inhibition of the pathway is achieved both by inactivating mutations and epigenetic silencing, c) Re-activation of the pathway can inhibit the growth of mouse CMML in vivo and human AML in vitro. In this application we: a) Assess the biological effects of Notch pathway re- activation in AML, b) Identify the molecular and epigenetic mode of pathway inhibition in human AML and c) Study co-operation of Notch pathway mutations with additional genomic lesions and their effect in AML initiation and progression. We strongly believe that these studies can lead to future targeted therapies of AML and related myeloid neoplasms, as re-activation of the Notch pathway can be achieved using both peptide and antibody agonist approaches.