Researches in recent years indicated that dinucleotide biases can affect codon bias [20] . To study the possible effect of dinucleotide composition on codon usage of the PCV genome, the relative abundances of the 16 dinucleotides in genome of the 28 PCV strains were calculated ( Table 5 ). The result show that the occurrences of dinucleotides are not randomly distributed and no dinucleotides were present at the expected frequencies.