To examine the reason for N c variation under the same GC3 content, the relationship between the GC1, GC2 and GC3 contents for AV1, AC1 and AC4 genes was further examined (Fig.2b) . It was observed that the GC1 content was always higher than GC2 content for the AV1 and AC1 genes, whereas the GC1 content of the AC4 gene was generally lower than the GC2 content. This result was coincident with the base composition analysis (Table 1) . Thus the variations of the synonymous first and second codon positions for the AC4 gene might be caused by the translational selection utilizing G or C at the synonymous second position.