Members of the bocavirus genus contain an 3 rd ORF of unknown function labeled NP1 gene. Recently another parvovirus (PPV4) was identified in porcine feces that also contained a central 3 rd ORF, although unrelated in sequence to the bocaviruses NP1 [59] . None of the ORFs of PPV4 clustered phylogenetically with the bocaviruses (data not shown) but instead clustered with members of the parvovirus genus [59] . PPV4 is therefore unrelated to the bocaviruses reported here. HBoV1 has been associated with respiratory symptoms while other HBoV may be associated with diarrhea and acute flaccid paralysis [5, [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] 25] . A gorilla bocavirus was detected in captive animals in the US experiencing severe diarrhea [6] . PBoV1 was found in pigs with PMWS in Sweden [27] and in pigs with respiratory tract symptoms in China [58] . In the present study, both PBoV1 and PBoV2 were highly prevalent in both asymptomatic swine from five provinces of China. Further studies are needed to examine possible associations between infections with these different porcine bocaviruses, the viral loads excreted, the presence of co-infections and various porcine diseases.  Analysis of codon usage and nucleotide composition bias in polioviruses BACKGROUND: Poliovirus, the causative agent of poliomyelitis, is a human enterovirus and a member of the family of Picornaviridae and among the most rapidly evolving viruses known. Analysis of codon usage can reveal much about the molecular evolution of the viruses. However, little information about synonymous codon usage pattern of polioviruses genome has been acquired to date. METHODS: The relative synonymous codon usage (RSCU) values, effective number of codon (ENC) values, nucleotide contents and dinucleotides were investigated and a comparative analysis of codon usage pattern for open reading frames (ORFs) among 48 polioviruses isolates including 31 of genotype 1, 13 of genotype 2 and 4 of genotype 3. RESULTS: The result shows that the overall extent of codon usage bias in poliovirus samples is low (mean ENC = 53.754 > 40). The general correlation between base composition and codon usage bias suggests that mutational pressure rather than natural selection is the main factor that determines the codon usage bias in those polioviruses. Depending on the RSCU data, it was found that there was a significant variation in bias of codon usage among three genotypes. Geographic factor also has some effect on the codon usage pattern (exists in the genotype-1 of polioviruses). No significant effect in gene length or vaccine derived polioviruses (DVPVs), wild viruses and live attenuated virus was observed on the variations of synonymous codon usage in the virus genes. The relative abundance of dinucleotide (CpG) in the ORFs of polioviruses are far below expected values especially in DVPVs and attenuated virus of polioviruses genotype 1. CONCLUSION: The information from this study may not only have theoretical value in understanding poliovirus evolution, especially for DVPVs genotype 1, but also have potential value for the development of poliovirus vaccines. The result shows that the overall extent of codon usage bias in poliovirus samples is low (mean ENC = 53.754 > 40). The general correlation between base composition and codon usage bias suggests that mutational pressure rather than natural selection is the main factor that determines the codon usage bias in those polioviruses. Depending on the RSCU data, it was found that there was a significant variation in bias of codon usage among three genotypes. Geographic factor also has some effect on the codon usage pattern (exists in the genotype-1 of polioviruses). No significant effect in gene length or vaccine derived polioviruses (DVPVs), wild viruses and live attenuated virus was observed on the variations of synonymous codon usage in the virus genes. The relative abundance of dinucleotide (CpG) in the ORFs of polioviruses are far below expected values especially in DVPVs and attenuated virus of polioviruses genotype 1. Conclusion: The information from this study may not only have theoretical value in understanding poliovirus evolution, especially for DVPVs genotype 1, but also have potential value for the development of poliovirus vaccines.