Animal RNA viruses are [http://www.ncbi.nlm.nih.gov/ICTVdb/Ictv/fr-fst-g.htm classified] into three distinct groups depending on their genome and mode of replication ( and the numerical groups based on the older [[ Virus_classification#Baltimore_classification|Baltimore classification]]):
*[[Double-stranded RNA viruses]] (Group III) contain from one to a dozen different RNA molecules, each of which codes for one or more viral proteins.
*[[#Group IV - positive-sense ssRNA viruses|Positive-sense ssRNA viruses]] (Group IV) have their genome directly utilized as if it were mRNA, with host [[ribosomes]] translating it into a single protein which is modified by host and viral proteins to form the various proteins needed for replication. One of these includes [[RNA-dependent RNA polymerase]] (RNA replicase), which copies the viral RNA to form a double-stranded replicative form, in turn this directs the formation of new virions.
*[[#Group V - negative-sense ssRNA viruses|Negative-sense ssRNA viruses]] (Group V) must have their genome copied by an [[RNA-dependent RNA polymerase]] to form positive-sense RNA. This means that the virus must bring along with it the RNA replicase enzyme. The positive-sense RNA molecule then acts as viral mRNA, which is translated into proteins by the host [[ribosomes]]. The resultant protein goes on to direct the synthesis of new virions, such as [[capsid]] proteins and RNA replicase, which is used to produce new negative-sense RNA molecules.
[[Retrovirus]]es (Group VI) have a single-stranded RNA genome but are generally not considered RNA viruses because they use DNA intermediates to replicate. [[Reverse transcriptase]], a viral enzyme that comes from the virus itself after it is uncoated, converts the viral RNA into a complementary strand of DNA, which is copied to produce a double stranded molecule of viral DNA. After this DNA is integrated into the host genome using the viral enzyme [[integrase]], expression of the encoded genes may lead to the formation of new virions.