The information of 28 PCV genomes, including the genotype, length value, the isolated area and GenBank accession numbers of these strains was listed in the Table 6 . In order to compare the differences between PCV and its host, twenty swine gene were gained and detailed information of these genes is listed in Table 7 . In addition, to compare the codon usage patterns among different viruses, twenty-five viral genomes of Circoviridae were taken into account ( Table 8 ). All of the sequences were downloaded from NCBI (http:// www.ncbi.nlm.nih.gov/Genbank/). Each general nucleotide composition (T%,A%,C% and G%) and each nucleotide composition in the third site of codon (T 3 %,A 3 %, C 3 % and G 3 %) in PCV coding sequence were calculated by biosoftware DNAStar7.0 for windows.