'Viral RNA-dependent RNA polymerases do not have a proofreading nuclease activity, which leads to high rates of misincorporation of nucleotides during genome replication. The mutation rates are estimated to be on the order of one mutation per 10^3–10^7 nucleotides, resulting in approximately one error per replicated genome (Drake 1991). In comparison, the mutation rate in E. coli or S. cerevisiae, where cellular polymerases benefit from an error-correcting mechanism, is on the order of one mutation per 10^9–10^10 nucleotides.'