“…Certain case studies have evidenced that the selective pressure plays an important role on the CUB of viral genes, for example, duck and human hepatitis viruses , small non-enveloped viruses (Shi et al, 2013), foot-and-mouth disease virus (Zhou et al, 2010), HIV (Pandit & Sinha, 2011), etc. In addition, the mutation pressure, genomic architecture, and secondary structure of genes could also constrain the codon usages of virus (Cardinale et al, 2013;Hershberg & Petrov, 2008;Sharp et al, 1993). For example, the variation in the number of hydrogen bonds between codons GAA and GAG for Glu (synonymous transition substitution) distinctive in clade 1 and clade 2, respectively (Table 4), could affect the formation of short doublestranded helices.…”