Werner Syndrome (WS) is an autosomal recessive condition characterized by an early onset of age-related symptoms. WS patients also experience an increased risk of rare non-epithelial cancers, especially mesenchymalneoplasms such as sarcomas. Fibroblasts isolated from WS cells senesce prematurely in culture and display increased chromosomal aberrations. WRN, the protein mutated in WS, is unique among the RecQ family proteins possesses on exonuclease activity and 32 to 52 helicase in a single polypeptide. There is accumulating evidence suggesting that WRN contributes to the maintenance of genomic integrity through its involvement in various DNA damage repair pathways and plays a role in telomere maintenance. However, the mechanism by which WRN functions in DNA repair, especially in DNA double-strand break (DSB) repair is still elusive. In vitro and indirect evidence leads to the conclusion that WRN may play a role both in homologous recombination (HR) as well as nonhomologous end joining (NHEJ). Recently, evidence showed that WRN is recruited to DNA damage sites and phosphorylated by PIKK3 kinase family in response to DNA DSB. In this proposal, we will test the hypothesis that WRN is recruited to DNA double-strand breaks (DSB) in vivo, to determine the function of WRN phosphorylation, and its involvement in the process of NHEJ or HR in response to DNA damage. Our specific aims are: (1) To determine the mechanism by which WRN is recruited to the sites of DNA double-strand breaks; (2) To test the hypothesis that WRN is phosphorylated by DNA-PK in response to DNA double-strand breaks and the phosphorylation status of WRN modulates its functions at DNA damage sites; and (3) To verify the hypothesis that WRN plays a role in nonhomologous end joining (NHEJ) and/or homologous recombination (HR) pathways of DNA double-strand break repair. Accomplishment of the proposed research would lead to the understanding of WRN's function in response to DNA damage and help further elucidate the role of WRN in cancer.