Werner syndrome helicase (WRN) plays important roles in DNA repair and the maintenance of genome integrity. Germline mutations in WRN give rise to Werner syndrome, a rare autosomal recessive progeroid syndrome that also features cancer predisposition. Interest in WRN as a therapeutic target has increased massively following the identification of WRN as the top synthetic lethal target for microsatellite instable (MSI) cancers. High throughput screens have identified candidate WRN helicase inhibitors, but the development of potent, selective inhibitors would be significantly enhanced by high-resolution structural information.. In this study we have further characterized the functions of WRN that are required for survival of MSI cancer cells, showing that ATP binding and hydrolysis are required for complementation of siRNA-mediated WRN silencing. A crystal structure of the WRN helicase core at 2.2 Å resolutionfeatures an atypical mode of nucleotide binding with extensive contacts formed by motif VI, which in turn defines the relative positioning of the two RecA like domains. The structure features a novel additional β-hairpin in the second RecA and an unusual helical hairpin in the Zn 2+ binding domain; modelling DNA substrates based on existing RecQ DNA complexes suggests roles for these features in the binding of alternative DNA structures..