Studies of synonymous codon usage in viruses can reveal much about viral genomes [25] . The overall codon usage among 48 ORFs of polioviruses was analyzed in this study. First, the ENC values of all the poliovirus samples were analyzed, and the results showed that the majority of polioviruses do not have a strong codon bias (mean ENC = 53.754 > 40). In addition, together with published data on codon usage bias among some RNA viruses, such as BVDV, H5N1 Table 4 Correlation analysis between the A, U, C, G contents and A 3 , U 3 , C 3 , G 3 influenza virus and SARS-covs with mean values of 51.43, 50.91 and 48.99, respectively, one possible explanation for this is that the weak codon bias of RNA virus is advantageous to replicate efficiently in vertebrate host cells, with potentially distinct codon preferences [26] [27] [28] . Natural selection and mutation pressure are thought to be the main factors that account for codon usage variation in different organisms [29] [30] [31] . In this study, the general association between codon usage bias and base composition suggests that mutational pressure, rather than natural selection is the mainly factors on codon usage pattern of polioviruses.