Os [[retrovirus]] (Grupo VI) teñen un xenoma de ARN monocatenario pero non se consideran xeralmente dentro do grupo dos virus de ARN porque utilizan intermediatos de ADN para replicarse. O encima viral [[reversotranscritase]], que se encontra no interior do virus e se libera cando este se decapsida, converte o ARN viral nunha fibra complementaria de ADN, que despois se copia para producir a cadea complementaria orixinando unha molécula de ADN viral bicatenario. Despois este ADN intégrase nun cromosoma do hóspede utilizando o encima viral [[integrase]]. A expresión dos xenes codificados nese ADN integrado pode dar lugar á formación de novos virións.
Classification of the RNA viruses has proven to be a difficult problem. This is in part due to the high mutation rates these genomes undergo. Classification is based principally on the type of genome (double stranded, negative or positive single strand) and gene number and organisation. Currently there are 5 orders and 47 families of RNA viruses recognised. There are also a number of unassigned species and genera.
Related to but distinct from the RNA viruses are the [[viroid]]s and the [[Satellite (biology)|RNA satellite viruses]]. These are not currently classified as RNA viruses and are described on their own pages.
Positive strand RNA viruses===
This is the single largest group of RNA viruses with 30 families. Attempts have been made to group these families in higher orders. These proposals were based on an analysis of the RNA polymerases and are still under consideration. To date the suggestions proposed have not been broadly accepted because of doubts over the suitability of a single gene to determine the taxonomy of the clade.