Because dinucleotide biases can affect codon bias, the relative abundance of dinucleotides in the coding regions of polioviruse genomes was assessed using the method described by Karlin and Burge [24] . A comparison of actual and expected dinucleotide frequencies of the 16 dinucleotides in coding region of the 48 polioviruses genomes was also undertaken. The odds ratio r xy =ƒ xy /ƒ y ƒ x , where ƒ x denotes the frequency of the nucleotide X, ƒ y denotes the frequency of the nucleotide Y, ƒ y ƒ x the expected frequency of the dinucleotide XY and ƒ xy the frequency of the dinucleotide XY, etc., for each dinucleotide were calculated. As a conservative criterion, for r xy > 1.23 (or < 0.78), the XY pair is considered to be of over-represented (or under-represented) relative abundance compared with a random association of mononucleotides.