The LPC strain was obtained from the high-virulence strain ALD after more than 1000 passages in rabbits. The two strains belong to subgroup 1.1, but significant difference in pathogenesis. To understand the pathogenesis of CSFV and study whether the virulence factor existed, construction of the ALD strain full length cDNA will help to investigate the differences in 3'-UTR between ALD and LPC strains which may affect viral replication. The full length cDNA will be used to investigate the role of NS5B from different viruses in viral replication and to elucidate the genetic determinants of pathogenesis in CSFV. Further analysis of the pathogenesis and the relationship between genes and virulence will be performed.