Abstract: In order to understand genetic characters of ORF5 and ORF7 gene of Porcine reproductive and respiratory syndrome virus（PRRSV） at the molecular level, and provide information for genetic mutation and virus evolution research, PRRSV samples from Ningxia were collected and ORF5, ORF7 gene of PRRSV were cloned by reverse transcription polymerase chain reaction (RT\|PCR) method, then their homology alignments and evolution analysis were carried out using DNASTAR biology software. The homology analysis at the nucleotide and amino acid levels indicated that ORF5, ORF7 gene of PRRSV NX-1/2008 strain were highly homologous to the highly pathogenic PRRSV strain JXa-1/2007, especially ORF7 gene, whose homology was about 100%. The ORF5 gene of NX-1/2008 strain had a dot mutation with JXa-1/2007 in neutralization antibody determine site 196 amino acid. The space distance between NX-1/2008 strain and JXa-1/2007 was bigger than that between NX-1/2008 and HUB-1/2006, as well as NX-1/2008 and CH-1a/2001 strain, so this mutation did not occur among them. Compared with CH-1a/2001 strain, the immunogenic core B site of NX-1/2008 mutated at 39 amino acid, and the mutation was F to I. Meanwhile, the 33 amino acid, a glycosylation site played an important role in inducing neutralization antibody, mutated from N to S. In addition, the 32, 33 and 35 amino acids of deceit site A were also mutated. Sixty-nine percent of the mutation between NX-1 ORF7 gene and national low pathogenicity strains such as CH-1a/2001 was located in antigenic determinant sites and 56% of them were highly pathogenic strains mutated initiatively. These results indicated that there was larger difference in genome between highly and mildly pathogenic PRRSV strains and the variation might be located in immunodominant region．

Key words: Porcine reproductive and respiratory syndrome virus, ORF5, ORF7, RT-PCR