acts as viral mRNA, which is translated into proteins by the host ribosomes. Retroviruses (Group VI) have a single-stranded RNA genome but, in general, are 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. Recombination Numerous RNA viruses are capable of genetic recombination when at least two viral genomes are present in the same host cell. Very rarely viral RNA can recombine with host RNA. RNA recombination appears to be a major driving force in determining genome architecture and the course of viral evolution among Picornaviridae ((+)ssRNA), e.g. poliovirus. In the Retroviridae ((+)ssRNA), e.g. HIV, damage in the RNA genome appears to be avoided during reverse transcription by strand switching, a form of recombination. Recombination also occurs in the Reoviridae (dsRNA), e.g. reovirus; Orthomyxoviridae ((-)ssRNA), e.g. influenza virus; and Coronaviridae ((+)ssRNA), e.g. SARS. Recombination in RNA viruses appears to be an adaptation for coping with genome damage. Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans. Classification Classification of the RNA viruses is difficult. 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 organization. Currently, there are 5 orders and 47 families of RNA viruses recognized. There are also many unassigned species and genera. Related to but distinct from the RNA viruses are the viroids and the RNA satellite viruses. These are not currently classified as RNA viruses and are described on their own pages. A study of several thousand RNA viruses has shown the presence of at least five main taxa: a levivirus and relatives group; a picornavirus supergroup; an alphavirus supergroup plus a flavivirus supergroup; the dsRNA viruses; and the -ve strand viruses. The lentivirus group appears to be basal to all the remaining RNA viruses. The next major division lies between the picornasupragroup and the remaining viruses. The dsRNA viruses appear to