No unusual variation of G/C or purine/pyrimidine composition was observed at the third codon position or at positions one or two of the codon (not shown). Likewise, we analyzed the dinucleotide composition at all three possible positions. Although there was a general under-representation of CpG and UpA, and overrepresentation of CpA and UpG, there was no correlation between areas of SSSV and regions of unusual dinucleotide frequencies (data not shown). These results indicated that evolutionary constraints restrict nucleotide variation within the 5 0 -coding regions of flavivirus genomes.