Among the various notable genomic features and properties in the novel bat SaV, we were most intrigued by its remarkably high G+C genomic content. Most caliciviruses have a genomic G+C content of 44.2-57.4 mol%. Among them, the genomic G+C content of the SaVs lie within the relatively narrow range of 49.0-53.6 mol% in spite of their genetic diversity. Hence, the presently observed G+C genomic content of 60.2% is significantly higher than that for other SaVs or caliciviruses, and indeed would rank amongst mammalian RNA viruses with the highest G+C genomic content [43] . Relatively little is known about the evolution of genome composition in caliciviruses. A number of factors have been postulated to exert selectional pressure on the G+C content of viral genomes, including host body temperature, immune pressure, codon and nucleotide usage patterns [44] [45] [46] [47] . Our results suggested that the increased G+C content is associated with a decrease in CpG suppression, but does not have a direct correlation with codon usage bias. We are unaware of any Table 3 . Comparison of genome identities and amino acid identities between the predicted polyproteins of bat SaV and the selected SaV. previous findings indicating that genomes of bat viruses are under less CpG suppression, thus the observed reduction in CpG suppresion is unlikely to result from host-related factors. The greater codon usage bias in the bat SaV genome is another interesting genome feature, which could be associated with altered dinucleotide frequencies. The association could be tested by Markov modelling of the dinucleotide and codon frequencies in the SaV genomes, although the small genome sizes and the presently small number of complete genomes would limit the usefulness of this approach [48] . The novel SaV described presently is the first known member of the Caliciviridae in bats. The approach to its discovery is based on the established strategy of targeted genetic screening informed by conserved sequences of related viruses. Although this ''homologybased'' strategy has been successful in the discovery of numerous viruses, the advent of affordable high-density microarrays and high-thoughput sequencing has given rise to virus discovery through metagenomics. Indeed, the first canine SaVs were discovered recently by metagenome sequencing of canine diarrhoea samples on a high-throughput pyrosequencer [23] . Important advantages of the new method include detection of novel viruses not closely related to known viruses, and the capacity to detect multiple divergent viruses in cases of co-infection. However, metagenomics sequencing can suffer from possible bias during sample preparation [49] , and it is unlikely to detect very low titres of viruses in a specimen, such as the three bat faecal samples that were positive upon repeat screening with specific PCR primers in the present study. While we anticipate the increasing utilization of the metagenomics approach, existing methods such as viral culture, electron microscopy and targeted nucleic acid amplification would continue to serve important roles in virus discovery.