In order to examine the base composition variation among different genes, the base composition of different protein-coding genes was calculated. Table  1 shows that with the exception of the AC3 gene that has a lower average GC content (0.391), no obvious difference in GC content was found among other tested genes. However, differences in GC content at the different synonymous positions of codons were apparent. For example, GC content at the synonymous first position of codons for the AV2 gene is 0.563, while that for the AC4 gene is only 0.412. GC content at the synonymous third position of codons for the AC4 gene is 0.534, while that for the BV1 gene is only 0.399.