Project Summary The goal of this proposal is to investigate the mechanism by which alteration in the telomere binding protein POT1 results in tumorigenesis. Mutations in POT1 (>100) have been associated with several types of human cancers such as chronic lymphocytic leukemia (CLL), cutaneous T cell lymphomas (CTCL), mantel cell lymphomas, parathyroid adenomas, gliomas, and melanomas. Our preliminary data indicate that perturbations in POT1 lead to replication stress at telomeres and telomere dysfunction. Moreover, we have shown that inhibition of POT1 in the common lymphoid progenitor cells (CLPs) promotes the formation of thymic lymphomas in mice. Here we will test our hypothesis that POT1 mutations trigger telomere replication stress which in turn leads to genome instability and cancer development. In addition, we will address the mechanism that enables cancer cells with POT1 mutations to bypass telomere replication stress and proliferate indefinitely. Lastly, we will develop a genetic screen to interrogate POT1 mutations in vivo and identify driver-mutations. In conclusion, our study will provide an important step towards identifying novel and effective therapeutic targets to treat an increasing number of cancer patients with POT1 mutations.