The origin and evolution of viruses is a topic of debate; in particular, the timing and relationship of cellular and viral evolution are controversial. To identify cellular homologues of viral proteins, Krupovic and Koonin performed an analysis of sequences and structures of capsid and nucleocapsid proteins from representative members of 135 virus taxa. The authors identified 20 architectural classes of viral capsid and nucleocapsid proteins, which were hypothesized to have evolved independently. A systematic comparison of viral proteins with a database of protein sequences and structures from cellular organisms identified substantial similarities between viral and cellular proteins. Virus genomes were also found to contain distinct structural modules that have different evolutionary paths. The authors argue that these observations support a view that viruses evolved on several, indepdendent occasions by co-opting host cellular proteins to become part of virions.