"RNA viruses are the fastest evolving entities in nature. Such rapid evolution is explained by their mutation rates, which are orders of magnitude higher than those of DNA organisms. The error-prone replication of RNA viruses also has public-health implications related to pathogenesis, antiviral research, or viral emergence. However, current mutation rate estimates vary by more than 100-fold across different RNA viruses and sometimes over one order of magnitude for the same virus, and the causes of this variation remain poorly understood. Here, we plan to investigate variability in the mutation rate of RNA viruses at different levels. First, polymorphic sites in viral polymerases could produce populations with heterogeneous mutation rates, and the spread of genotypes with altered mutation rates might influence viral fitness and disease progression. Second, spontaneous mutations might occur preferentially at some regions of the viral genome, and these mutational hotspots could match genome regions where the selective pressure imposed by the host is strongest, thereby increasing viral adaptability. Third, RNA viruses with different types of genomes, such as single-stranded versus double-stranded RNA or sense versus anti-sense genome polarity might differ in their susceptibility to nucleic-acid damage or host-mediated editing and thus, in their mutation rate. Fourth, RNA viruses with larger genomes might have evolved increased replication fidelity to compensate for their greater genetic load. To address these issues, we will use several biomedically relevant and/or model viruses, including HIV-1, hepatitis C virus, a rotavirus, a coronavirus and a bacteriophage. The experimental procedures will include in vitro replication fidelity assays, ex vivo infections using cell cultures, and analysis of patient samples by next-generation sequencing. This thorough and multilevel approach may reveal previously unrecognized mechanisms for generating diversity in RNA viruses."