A viral DNA form that survives extremes The prokaryote Sulfolobus islandicus lives at extreme temperatures (∼80°C) and acidity (pH 3). It is infected by the rudivirus SIRV2. DiMaio et al. determined the structure of the SIRV2 virus using cryo–electron microscopy to understand how the virus survives these brutal conditions. Most DNA in nature assumes a B-form shape. The virion, on the other hand, contains highly unusual A-form DNA that may help it survive adverse conditions. The viral capsid protein forms an extended α-helical structure that wraps around the viral DNA, possibly stabilizing the A-form DNA. Science , this issue p. 914