An RNA virus is a virus that either uses RNA as its genetic material, or whose genetic material passes through an RNA intermediate during replication. For example, Hepatitis B virus is classified as an RNA virus, even though its genome is double-stranded DNA, because the genome is transcribed into RNA during replication. The basis for this classification is error-prone RNA replication: All RNA viruses have very high mutation rates because they lack DNA polymerases which can find and edit out mistakes. DNA viruses have considerably lower mutation rates. See also retroviruses.
Although RNA usually mutates rapidly, recent work found that the SARS virus and related RNA viruses contain a gene that mutates very slowly. The gene in question has a complex three-dimensional structure which is hypothesized to provide a chemical function necessary for viral propagation. (See ribozyme.) If so, most mutations would render it unfit for that purpose and would not propagate.
Some RNA viruses: