The set of reference sequences used to calculate CAI values in this study were the genes coding for coat proteins. According to the calculated CAI values, 5% of the total genes with extremely high and low CAI values were regarded as the high and low dataset, respectively. Then we calculated the average RSCU values of the two gene samples and subtracted them subsequently in each dataset group (ΔRSCU). If the ΔRSCU values are larger than 0.08, then this codon will be defined as the optimal codon (Duret and Mouchiroud, 1999) .