Virus-induced interferon formation depends on the presence within the cell of a viral ribonucleic acid. This RNA may either be double stranded or, in certain cases, single stranded. The double-stranded RNA can be derived from a virus, such as reovirus, which contains this type of RNA, or it may be synthesized within the cell using viral single-stranded RNA as a template. Single-stranded RNA must possess a stable configuration in solution to be active, and certain viral RNA molecules appear to be active for this reason. The presence of this RNA triggers a derepression event, which is probably nuclear, by an unknown mechanism, and this is followed by the production of an interferon messenger RNA and its translation. Little is known of the derepression event or the events that follow it.