Flaviviruses, including dengue and West Nile viruses, have an RNA genome that encodes a single polyprotein that is processed into three structural and seven non-structural proteins inside the host cell. Six of the non-structural proteins form an ER membrane–associated replicative complex. The remaining non-structural protein, NS1, is highly conserved and essential for viral replication. NS1 is known to be lipid associated, localizing with the viral replicative complex as a dimer during early stages of infection and interacting with the host complement system when secreted in a hexameric form, but its exact function is unknown. To address this, Smith and colleagues have determined the crystal structures of glycosylated NS1 from West Nile virus and dengue virus type 2. The structures of the NS1 proteins are quite similar. NS1 forms a dimer centered around an extended β-sheet. Each monomer is composed of a β-roll domain involved in dimerization, a wing domain that can be divided into connector and α/β subdomains, and a core β-ladder domain that contributes nine rungs to the extended central β-sheet. The connector subdomain and β-roll form a hydrophobic surface that extends from the dimer and is a good candidate for membrane interactions. Mutations within the connector subdomain permitted liposome remodeling but affected viral replication, suggesting that interactions with the viral replicative complex were compromised. NS1 crystallized as trimer of dimers, and the resulting hexamer fits well with two-dimensional EM class averages of NS1. In the hexamer, the β-rolls line an interior central cavity, and antibody epitopes map to the exterior surface in the wing and β-ladder domains. Although the structures reveal distinct regions for membrane association and immune system interactions, insight into how these features contribute to pathogenicity await further study. (Science doi:10.1126/science.1247749, 6 February 2014)