ABSTRACT
It is thought that RNA is only used as the primary repository of genetic information by a limited and heterogeneous assortment of viruses. This characteristic sets the relevant groups of viruses apart from all other living forms, and raises the evolutionary question of how such a distinction arose. Despite its intrinsic interest, this large question has attracted only passing interest from virologists because, in common with most questions of ultimate origins, it seemed unlikely that we would ever know. Nevertheless, the question remains an intriguing one and has attracted a certain amount of speculation.
