Rotaviruses, like all viruses, reproduce inside living cells. Making new viruses requires assembling replication factories via a complex, little known process that involves both viral and cellular components. A report in the Proceedings of the National Academy of Sciences by a multidisciplinary team led by researchers at Baylor College of Medicine reveals that the formation of rotavirus factories depends on a cellular protein called CK1α, which chemically modifies viral component NSP2, thus triggering its localization and assembly into the virus factory, an essential step in the formation of new viruses.